8 Rivers, Lakes, and Landscapes

This chapter is a pre-review version

In this chapter, we look at water on the surface of the continents, including rivers, lakes, wetlands etc. As we’ve seen from the section on the water cycle, about 40,000 cubic kilometers of water runs over the surfaces of the continents per year, and the total amount of surface water is around 190,000 cubic kilometers at any one time time.

Rivers have huge impacts on the landscape over time. They erode the channels and valleys in which they flow; they transport sediments supplied by weathering and erosion; and they deposit sediment in a variety of depositional landforms. Rivers are vitally important to humans as sources of water for drinking, agriculture, and industry, as sources of food, and as means of transportation.

Learning outcomes:

By the end of this section you should be able to:

  • Define overland flow, base flow, stream flow, and discharge;
  • Describe major features of drainage basins and river systems;
  • dentify processes and features of erosion and deposition in river systems;
  • Evaluate flooding risks associated with rivers;
  • Interpret the formation of open and closed lakes.

Overland and stream flow

Onland water starts its journey as rain or snow falling on the land surface. Snow may be locked up in glaciers for periods lasting from hours to millions of years, but eventually it joins rainwater on the surface of the land. If we look at new rainfall or meltwater, we can distinguish water that travels as unconfined sheets — overland flow — from water that travels in confined channels — stream flow.

Overland flow

Overland flow or surface runoff. Public Domain, https://commons.wikimedia.org/w/index.php?curid=761208

Overland flow doesn’t start immediately when rain falls or ice melts on the landscape. The first water in most cases soaks in to any porous material — regolith, soil, or porous rock — that is present at the surface of the Geosphere. The soaked-in water becomes groundwater, the subject of a later section. Once the ground is saturated, if the supply of rain or meltwater is sufficient, sheets of water start to accumulate on the land surface and move down-slope towards lower ground. This is overland flow. In most areas, it requires very heavy rain for overland flow to be visible, but in built-up areas where the ground is covered with concrete or asphalt, and in areas of naturally impermeable rock, overland flow may occur during moderately heavy rain.

Stream flow

In most environments, however, overland flow is fastest on locally steep slopes, and that leads to erosion, further concentrating the flow into gullies — small valleys — marking the transition to channellized or stream flow.

Water supply to streams

Overland flow and base flow. ©CC-BY-NC-SA The Open University. https://www.open.edu/openlearn/pluginfile.php/66016/mod_oucontent/oucontent/453/49e96c09/5029a34d/s278_14_f003hi.jpg https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode

When we look at a stream it may not immediately be clear where the water is coming from. Certainly, overland flow is important in feeding many streams, but additional water — groundwater — enters most streams through the channel sides and channel flow. This component of water, that initially soaks in to the ground and enters a stream through its base, is known as base flow.

Therefore the total flow of water through a stream is approximately the sum of the amount entering as overland flow and the amount entering as base flow[1].

The rate of flow in a channel is determined by a number of factors, including the slope or gradient of the channel, the depth of water, and the roughness of the channel floor and sides.

Channel gradients

Channel gradients are extremely variable. Although they could be measured in degrees, many channels have extremely gentle slopes (fractions of a degree) so that gradients are more often measured as “metres per kilometre” or (particularly in the USA) in “feet per mile”.

Four ways of measuring the gradient of a river: metres per kilometre; feet per mile; angle in degrees; gradient as a percentage.

For example, the North Saskatchewan River in Edmonton, AB, Canada, where this text is being written, has a gradent of about 0.3 m/km which translates to about 0.17°. However, gradients are extremely variable. The steepest mountain streams may have gradients as high as 45°, or 1000 m/km. In contrast, downstream sections of the Mississippi have gradients around 0.01 m/km or 0.0006°.

In most river systems, the gradient is steep near the source, and becomes gentler downstream.

Flow velocity

The velocity of flow in streams is also very variable, both from stream to stream and even within a single channel.  In a straight channel, the rate of flow tends to be fastest slightly below the water surface in the middle of the channel. The rate of flow is less near the bed of the channel because of drag against the channel floor, due to the viscosity of water and the roughness of the bed. The force per unit area, or stress exerted on the bed by the water, which moves sediment as bed load, depends on the velocity gradient (i.e. how much the speed of the water changes with depth). As a result, shallow, fast-moving streams can move large cobbles and boulders, whereas deep or slow moving streams are less likely to move large fragments of rock.

If a channel is sinuous (i.e. it has many bends) the momentum of the water causes it to flow fastest on the outside curve of each bend, and more slowly on the inside curve. Erosion tends to occur where the flow is fastest, on the outer bank, while deposition mainly takes place from slow-moving water on the inside of a bend.

Plan view and cross-sections of a sinuous stream showing locations of fastest and slowest flow. Steve Earle cc CC BY 4.0. https://opentextbc.ca/physicalgeology2ed/chapter/13-3-stream-erosion-and-deposition/

Discharge

Discharge of a stream. By Benjamin J. Burger – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=91115730

The amount of water that flows through a stream is called its discharge. Discharge is the volume of water that passes a given point on the stream in unit time: it’s usually given in cubic metres per second or m3/s or sometimes  cubic kilometres per year km3/yr. If the average velocity of flow is known, then

discharge = velocity × cross-sectional area of stream

Q = vA

or

discharge = velocity × width × average depth

The discharge of the North Saskatchewan River in Edmonton, Alberta, is about 7 km3/yr or 220 m3/s[2].

A stream hydrograph. Increases in stream flow follow rainfall or snowmelt. The gradual decay in flow after the peaks reflects diminishing supply from groundwater. By NOAA, Public Domain, https://commons.wikimedia.org/w/index.php?curid=80635212

However, discharge tends to vary dramatically over time, especially with changing seasons. Hydrographs are a graphs of river discharge at one point in a stream, plotted against time, typically over periods of one to many years.

Hydrographs often show that short-term events such as large storms or snowmelt in the spring can lead to high discharge over short periods of time.  These short pulses of high discharge may account for most of the sediment transport.

Drainage basins and river systems

To further explore the behaviour of streams, we need to look at how they are organized in river systems and drainage basins.

Small streams (tributaries) typically merge downstream into larger rivers. Smaller streams that flow into a larger rivers are known as tributaries of the larger river. The area drained by a major river and its tributaries is a drainage basin. Drainage basins are separated by drainage divides[3].

Drainage basins of part of North America. https://www.hydrosheds.org/products/hydrobasins. Lehner, B., Grill G. (2013). Global river hydrography and network routing: baseline data and new approaches to study the world’s large river systems. Hydrological Processes, 27(15): 2171–2186. https://doi.org/10.1002/hyp.9740. Copyright Conservation Biology Group http://www.consbio.org/

 

Major drainage divides of N. America, separating areas that drain into different oceans; also shows internally drained basins that have no river outlets. By Pfly – Own work, CC BY-SA 3.0, Major drainage divides of N America. https://commons.wikimedia.org/w/index.php?curid=12131177

As tributaries join downstream, their discharge merges, so the discharge of individual channels rises, while the number of channels decreases. There are some exceptions where humans have modified rivers. In the map below, the discharge of the main channel of the Colorado River decreases downstream, because of very large human withdrawals of water for irrigation. Most of the withdrawn water re-enters the atmosphere through evapo-transpiration rather than entering the Gulf of California as it would have done in the natural state.

Map showing rivers in the USA, plotted showing discharge in cubic feet per second. Note that the discharge of individual channels generally increases downstream. The unusual downstream decrease in the discharge of the Colorado River is due to human withdrawal of water, mainly for irrigation. https://pacinst.org/wp-content/uploads/2013/06/american_rivers_gage_adjusted-1024×853.jpg. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. © 2023 Pacific Institute.

Much of the extra discharge in the downstream direction is accommodated by increases in both the width and depth of the channels, increasing the cross-sectional area. The average velocity of flow also tends to increase downstream, but more gradually. Despite this increase in velocity, the velocity gradient decreases because the width and depth are much greater. Downstream portions of large rivers are much less able to move coarse sediment particles.

Other features change from upstream to downstream. The slope tends to decrease downstream. If we plot a longitudinal profile of a mature river, showing elevation vs. distance downstream, we may see a curve from steep gradients upstream to very low gradients as the river approaches its base level, the elevation at which the river enters the sea, and the velocity of the river falls to near zero.

Downstream variation in a typical river system. Trista L. Thornberry-Ehrlich, Colorado State University.
https://www.nps.gov/subjects/geology/images/3Zones_Fluvial_1204-2018_tte-01.jpg

In practice, many rivers show more complicated longitudinal profiles because of barriers to flow, such as resistant rock units, glaciers, or anthropogenic features such as dams. Such obstructions tend to form lakes which form local base levels along the course of the river. The longitudinal profile of the Colorado River in the USA shows a number of local base levels at lakes and dams.

Longitudinal profiles of the Columbia River and its major tributaries, showing locations of major dams and other obstacles. Redrawn after Portugal E.W. 2014: Linking temporal and spatial variability of millenial and decadal scale sediment yield to aquatic habitat in the Columbia River basin. MSc thesis, Utah State University.

Over long periods of time (thousands to millions of years), rivers tend to smooth out such interruptions, by eroding obstacles and depositing sediment in lakes, restoring a graded profile.

The ability of a river to erode and transport solid material is strongly influenced by the slope. This is because the slope controls shear stress or traction the on the base and sides of the channel. Steeply sloping streams, typical of the headwaters of a river system, are described as having high energy[4] and are able to erode and move large particles as bed load. Gently sloping streams have lower energy and are less able to move coarse sediment, leading to more deposition in downstream portions of a river system.

In the following sections we will look at erosion and deposition in turn.

Canyon in SW Türkiye. JWF Waldron CC BY-SA-NC 4.0

Erosion

Downcutting and valley generation

Erosion in the headwaters of a river system typically occurs at the base and sides of stream channels, leading to downcutting, the lowering of the floor of the channel as material is eroded. If the rocks are exceptionally strong, this may lead to the development of a slot canyon, a valley with almost vertical sides.  More usually, however, steepening of the valley sides makes them unstable, and gravity brings material down the valley sides into the channel. Such gravity-driven processes are known as mass wasting.

Slope processes and mass wasting

Mass wasting includes a huge variety of processes[5], many of which are of major concern to human populations. The more rapid processes may cause death and destruction of property, whereas even slow processes of mass wasting can cause serious damage and destruction to buildings, highways, and other human infrastructure built on or near slopes. Civil engineers pay a lot of attention to mass wasting processes.

Rockfalls

Rockfall on the Zion-Mount Carmel Highway, By Zion National Park – Public Domain, https://commons.wikimedia.org/w/index.php?curid=45373208

When rock falls from the steepest slopes, it moves downward through air under the action of gravity, either as single pieces, or as multiple fragments. Water is not involved in the mass transport process, although it typically plays a role in loosening the fragments before a fall.

Fallen rock material may accumulate at the base of a steep slope where it is known as scree or talus.

Slides

Translational slide. Highland & Bobrowsky,, 2008, The landslide handbook—A guide to understanding landslides: Reston, Virginia, U.S. Geological Survey Circular 1325

Coherent masses of rock or soil, that move above distinct failure surfaces, are known as slides. When the failure surface is approximately planar, the slide is known as a translational slide. Translational slides typically occur when there are planes of pre-existing weakness within the rocks, approximately parallel to the slope.

Rotational slide. Highland and Bobrowsky,, 2008, The landslide handbook—A guide to understanding landslides: Reston, Virginia, U.S. Geological Survey Circular 1325.

If the failure surface is curved, the slide is a rotational slide. Rotational slides can be recognized because the upper surface of the moved material is tilted inwards, toward the failure surface.

Rotational slide in Cusco, Peru, 2018. By Galeria del Ministerio de Defensa del Perú – https://www.flickr.com/photos/ministeriodedefensaperu/39935939755/in/dateposted/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=67408597

Flows

As material moves down-slope in a slide it may become progressively more disrupted. Such disrupted slides are sometimes called slumps although there is no sharp boundary between the two categories. With further disruption, the moving material may develop into a flow.  Flows are masses in which the moving material loses coherence as it moves, behaving more like a liquid. Typically there must be either water or air present between the solid particles of rock, in order for a flow to move.

Debris flows

If moving material is water-saturated, it may develop into a debris flow, earth flow, or mudflowin which all trace of the original structure is lost, and blocks of rock are carried along by a muddy matrix. Debris flows are some of the most hazardous mass-wasting phenomena as they may move fast (several tens of kilometres per hour) over large distances. Debris flows can carry blocks of rock that are metres or tens of metres across. Debris flows may be laminar (in which all the material is transported roughly parallel to the base of the flow) or turbulent (in which material is carried by swirling movements in the flow) like rivers, but the deposits of debris flows are typically very poorly sorted, containing a wide range of grain sizes.

Debris Flow in Los Ageles County, California, 2017. https://www.usgs.gov/media/images/las-lomas-debris-flow-january-2017-0

Debris flows are common on the slopes of volcanoes (covered in the section on igneous processes in the rock cycle) where they are known as Lahars.

Rock and debris avalanches

When a large volume of rock falls from a steep slope, collisions between fragments, and possibly the pressure of trapped air, may cause the falling debris to behave as a flow, which can continue moving on more gentle lower slopes. In some cases the runout of a debris avalanche may even extend some distance uphill on the opposite side of a valley, as happened in the case of the Frank Slide[6] in Alberta, Canada. In this event, a debris avalanche from Turtle Mountain obliterated part of the settler town of Frank, killing between 70 and 90 people and covering the floor of the Crowsnest river valley with debris, despite warnings from the Piikani Nation of the Blackfoot Confederacy. The events were described in the Canadian Mountain Assessment (2024) by Daniel Melting Tallow:

Piikani people have been in that area for thousands of years, and the Europeans came and found some coal in that area….The Piikani people were warning the people there: “Don’t live (there), don’t build your house. Build it further because that mountain is shaking” because they knew that. They didn’t listen to them: “Oh they’re just savages…” and stuff like that. They didn’t believe in their way of thinking and their knowledge and their knowing. Then they all settled in that area, and one night, the whole thing came down.  The mountain came down and it buried a whole town.  …..Underneath, the town is still there, and there are some bodies down there too. So our elders and our stories and our tales, they should be an addition to predicting what’s going to happen. Those Blackfoots knew what was going to happen… Canadian Mountain Assessment 2.6.1

The Frank Slide in April 1903 covered part of the town of Frank in debris that flowed in a debris avalanche from Turtle Mountain. The railway and highway were subsequently reconstructed across the debris field. Copyright © Google and sources listed within the image.

Creep

Trees on slopes affected by soil creep may develop bent trunks as their growth compensates for tilting. By Kent G. Budge – Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=112821053

Slow movements, millimetres per day or less, occur on slopes, where they are promoted by changes in the amount of water in soil. Expansion and contraction occurs as soil is moistened and dried, and with each small movement, grains may shift slightly down slope. The resulting slow movement is called creep.

Creep is particularly effective in areas where groundwater is repeatedly frozen and thawed. As ice forms in the pore spaces, it expands and displaces the slope upwards and outwards slightly. When the ice melts, the grains settle more vertically downwards. This type of creep is called solifluction.

Over periods of years, these slow processes can move material large distances down slopes into valleys.


River capture: In the upper diagram, the small stream at the left is undergoing rapid downcutting and headward erosion. In the lower diagram it has intercepted the waters of the larger river, leaving a dry valley on the right hand side. By en:user:Casito. – Created by en:user:Casito., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1248347

Headward erosion and stream piracy

Downcutting, combined with mass wasting, have unexpected consequences in the headwaters of some river systems. Rapidly downcutting streams oversteepen the region around their headwaters, causing  their valleys to extend in an upstream direction, a phenomenon called headward erosion. Headward erosion may cause one stream to cut into another’s valley, eventually diverting the water in a process called river capture or river piracyThe result may be a large valley with little or no water in it, a telltale sign that river capture has occured.

River capture at Deep Hollow, Nova Scotia; base map © Google.

Lateral erosion

Jordan River Valley, near the Dead Sea, 1937. The meandering river gradually widens its valley by lateral erosion at the edges of the meander belt. Current river flows have been much reduced by human activities. By Zoltan Kluger – digitool.haifa.ac.il, Public Domain, https://commons.wikimedia.org/w/index.php?curid=57660132

As streams approach their base level, downcutting becomes less and less effective. Rivers with low gradients carry out erosion by widening their valleys, rather than deepening them, a process called lateral erosion.Lateral erosion takes place particularly at bends in the course of rivers. Because of the behaviour of water in channels (see above), even a small bend in a river tends to be amplified over time, because the outer bank is eroded fastest. This process is an example of positive feedback — whereby a small change in a system leads to a process that amplifies that change. Lateral erosion gradually pushes out the sides of a valley, creating a broad floodplain in which both erosion and deposition may occur.

Depositional landforms

In the downstream parts of river systems, the velocity of water near the river bed decreases, and rivers become less able to transport sediment, which may be deposited in a variety of depositional landforms.

Alluvial fans

When a mountain stream emerges onto a broader valley floor, the velocity of flow may decrease abruptly, leading to the dumping of large amounts of sediment in an alluvial fan. In longitudinal river profiles, alluvial fans typically occur at a break in slope, and the deposition of the fan tends to smooth out the profile so that it more nearly approaches a graded profile. In an alluvial fan, the main channel of a river branches in a downstream direction, forming multiple distributariesAlluvial fans have a similar geometry to deltas. In both cases a river branches into multiple distributaries. Both are formed when flow in a river channel is abruptly slowed down. Alluvial fans form when that slowdown is due to a break in slope whereas deltas form where a river enters standing water — the sea or a lake. If a steeply flowing stream directly enters a standing body of water, the two process are combined and the result is called a fan delta. Typical sediments in alluvial fans are gravel and coarse sand.

Alluvial fan in the Zagros Mountains of southern Iran. https://earthobservatory.nasa.gov/images/36041/alluvial-fan-in-southern-iran

 

Fluvial deposits

The sediments deposited by rivers are described as fluvial. River systems that deposit sediment show enormous variety, but two end-member types of geometry are frequently seen: braided and meandering. Note that not all rivers can be slotted in to one of these categories; there are many intermediate types of rivers.

Braided systems

Braided rivers in Iceland. © Google and sources cited in image.

Braided river systems have multiple channels that branch and rejoin repeatedly downstream. Deposition takes place predominantly in bars that are located between the channels. During low discharge periods, flow is confined to the channels, but during flood stages the river covers the bars and deposits sediment on top of them too. Braiding in rivers seems to be encouraged by a very episodic flow, high bed load, and lack of vegetation. Braided rivers are common around glaciers, where summer melting supplies large volumes of sediment, and in arid areas with little vegetation.

Meandering systems

Meandering tributary of the Amazon, Brazil, showing ox-bow lakes. © Google and source cited in image.

Meandering river systems typically have a single channel active at any one time, but lateral erosion causes that channel to migrate, sometimes spectacularly, across a broad valley. The portions of the valley floor on either side of the channel make up the floodplain.

The behaviour of meandering channels is explained by a process of positive feedback whereby a small bend focusses the fastest flow, and therefore the most rapid erosion, on the outside of the curve, which in turn enhances the bend. Deposition, typically of sand, occurs on the inside of the curve where the current is slower, producing a point-bar. Bends which cause a large change in direction (more than about 90°) are called meanders.

Oxbow lake formation in a meandering stream.
Oxbow lake formation in a meandering stream. Phil Reiker, National Park Service Geologic Resources Division. Public domain. https://www.nps.gov/articles/meandering-stream.htm

Meander development does not continue indefinitely. Eventually a narrow meander neck develops, which may be eroded during a high discharge event, straightening the stream. The isolated meander channel then forms an ox-bow lake.

Floodplains

A flood occurs during high-discharge events, when a river’s discharge exceeds the capacity of its channel. The river spills out of the and expands over its floodplain. The rate of flow drops abruptly as water leaves the channel, and the fine-grained suspended load of the water is typically deposited across the floodplain. Floodplains are typically very fertile: in natural environments they have dense growth of plants. Humans exploit floodplains for agriculture and habitation; river channels are used by humans for transportation of goods. It's likely that plants contribute to the stabilization of river banks, thereby encouraging the development of meandering, rather than braided, rivers. Prior to the evolution of abundant land plants around 400 Ga, it appears that meandering rivers were less common.

Natural levées. Steven Earle CC B& 4.0. https://opentextbc.ca/geology/chapter/13-3-stream-erosion-and-deposition/

Rapid deposition at the edge of a channel may deposit a raised bank, called a natural levee (French levée). Levees are responsible for negative feedback, because they hinder further flooding. Humans living on floodplains have typically exploited levees by reinforcing them and raising them higher to prevent flooding.  However, under natural conditions, periodic flooding events distribute fine-grained sediment across floodplains, gradually raising them and contributing to their agricultural fertility. Construction of artificial levees locally reduces the frequency of flooding, and delivers more sediment downstream, where it may lead to new problems including obstruction of channels used for navigation, and increased flooding risk.

Flood risk may be assessed by examining hydrographs over decades or centuries, if data are available. A recurrence interval is the average period of time between two floods of the same magnitude. These intervals are determined by plotting the frequency of past floods using data gathered from the gauging stations over decades or longer. Sometimes, it may be possible to collect data from floodplain sediments that also help to determine flood risk.

Particular sizes of flood are characterized by their recurrence interval. For, example, a 50-year flood is one of a size that recurs on average every 50 years. In other words, there is a 2% chance that such a flood will occur in any given year. Note that because of the unpredictability of discharge, these estimates are averages. The actual intervals beteween "50-year" floods may be much shorter, or much longer.

An aerial view of Godavari River showing natural levées and an inundated floodplain taken from the IAF relief Helicopter on the border of Maharashtra and Andhra Pradesh, India on August 09, 2006. Cropped from an original by Ministry of Home Affairs (GODL-India), GODL-India, https://commons.wikimedia.org/w/index.php?curid=71562322

River mouths

When a river enters the sea, the velocity of channel flow decreases further, and flow may be modified by the action of waves and tides. Distinct depositional landforms called deltas and estuaries are developed. Because of the importance of the sea to their formation, these features are discussed in the chapter on seas and oceans.

Effects of base-level changes

Rivers erode laterally and by downcutting. Downcutting occurs particularly when sea-level falls (base level change), when the land surface is lifted (for example when isostatic rebound occurs following the melting of glaciers), when an obstacle is removed, or when discharge increases. Conversely, if sea-level rises, the land subsides, discharge decreases, or when a river is blocked by an obstacle, then sedimentation and/or lateral erosion are favoured. Changes in the history of a river system may leave their mark in the landscape.

Terraces

River terraces, Altai. By Heljqfy Alexei Rudoy - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10704339

Rivers that have been affected by repeated periods periods of lateral erosion or sedimentation  that have alternated with of downcutting show valleys with river terraces: flat areas part-way up the valley sides. Each terrace represents a pause in downcutting when the river has widenened its valley and floodplain. Renewed downcutting leaves parts of the abandoned floodplain on the valley sides.

 

Incised meanders

Incised meanders, Utah, USA. James St. John at https://flickr.com/photos/47445767@N05/49102339486 (archive). Llicensed cc-by-2.0.

When the base-level of a river falls dramatically relative to the valley floor, a river that has evolved into a laterally-eroding, meandering system may suddenly start downcutting again. Meanders that existed in the previous life of the river may become incised into the landscape by downcutting. In this way, meandering rivers may be preserved at the bottom of steep-sided or terraced valleys.

Superimposed drainage

The process of incision may produce a river channel that appears to have no relation to the bedrock features it flows through. For example, the Susquehanna River in Pennsylvania, U.S.A., shows a gently meandering shape typical of mature low-gradient rivers in broad floodplains, yet it cuts right through the "valley and ridge" province of the Appalachian mountains, formed of tilted and folded layers of erosion-resistant rock. The channel was presumably developed when the river flowed in a floodplain developed on flat-lying layers of younger rock like the present-day Mississippi system. Those younger layers have been removed by erosion, and downcutting has preserved a channel shape that is unrelated to the rocks through which it flows. Some smaller, younger tributaries of the Susquehanna do follow the geological structure; they presumably developed after the downcutting process.

Superimposed drainage: The Susquehanna River north of Harrisburg PA, U.S.A., has a channel that appears unrelated to the local geology of the Appalachian mountain belt, which is responsible for the dark forested ridges. The river was probably established on younger flat-lying rocks that have since been eroded away; downcutting then allowed it to cut through the resistant ridges of older rock. Within the modern channel  a braided system has developed locally. © 2023 Google and souces cited within the image.

Lakes

A lake is a standing body of water filling a depression on land.

Origin of lakes

Some sort of obstacle to flow is typically present to help produce a lake. For example:

  • Glaciers leave many erosional and depositional features that can obstruct water flow;
  • Volcanic activity forms craters and blocks rivers with lava dams;
  • Tectonic movements produce faults and deep rift valleys that may trap water in lakes;
  • Humans erect dams to provide irrigation water and generate power.

Lakes produced by any of these process may form local base levels in longitudinal river profiles.

Local base levels (represented by near-horizontal segments on these longitudinal profiles ) can be seen at dams and natural obstacles that have formed lakes on the Columbia River and its tributaries.  Redrawn after Portugal E.W. 2014: Linking temporal and spatial variability of millenial and decadal scale sediment yield to aquatic habitat in the Columbia River basin. MSc thesis, Utah State University.
Local base levels (represented by near-horizontal segments on these longitudinal profiles ) can be seen at dams and natural obstacles that have formed lakes on the Columbia River and its tributaries.  Redrawn after Portugal E.W. 2014: Linking temporal and spatial variability of millenial and decadal scale sediment yield to aquatic habitat in the Columbia River basin. MSc thesis, Utah State University.

In addition:

  • Rivers themselves may form lakes as part of their normal evolution, for example by cutting off meander channels leaving ox-bow lakes.

Most lakes are short-lived on geological time scales. The obstruction that produced a lake may be eroded away, while sediment and organic matter may accumulate in the lake, eventually filling them in. Some lakes transition over time into to wetlands. Some may evaporate away.

An importand variable that controls the behaviour of lakes over time is the nature of any outlet: the place where water leaves the lake. This characteristic is used to classify lakes into open and closed categories.

Open lakes

Open lakes are those which have an outlet with discharge comparable to the inflow that provides water to the lake. Typically the outlet is a stream, where the lake spills over a barrier into a river. Some open lakes have underground outlets into cave systems. Generally, their lake levels do not fluctuate much because when inflow increases, outflow also increases.

Because river water slows down dramatically as it enters a lake, coarse, bed-load sediment tends to be deposited near the lake shore. Fine grained muds settle out from the suspended load in offshore parts of the lake. The dissolved load travels mainly through the lake into the outlet, and most open lakes have relatively fresh water.

Moraine Lake in the Canadian Rockies, a typical open lake. As the name implies, the lake is retained by a natural glacial moraine, over which water exits. The intense colour of the water in summer is due to the suspended load of rock flour produced by glacial erosion, which scatters light. By Mike Boehmer - originally posted to Flickr as Moraine Lake, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=10569508

Closed lakes

Closed lakes have an inlet stream, but do not have an outlet stream. They tend to form in arid climates — places where evaporation is greater than precipitation. The water that enters the lake can only leave through evaporation, so dissolved ions derived from the dissolved load of the incoming river accumulate, and the lake becomes saline or salty. Water level fluctuates a great deal in most closed lakes, as a result of the changing balance between preciptiation and evaporation.

Oblique satellite view of Great Salt Lake, UT, U.S.A., a typical closed lake. The pale areas on the lake shoreline are exposed salt and other mineral deposits. The east-west boundary with the lake is due to a railway causway that limits mixing; the northern part of the lake has a higher suspended load of sediment. © Google and sources cited within the image.

The sediments formed in closed lakes typically include salt deposits formed by evaporation, in addition to the sands and muds also typical of open lakes.


  1. This statement neglects a small amount of interflow: water that travels in the vadose zone between the land surface and the water table. However, the rate of flow in the vadose zone is typically low. Groundwater concepts are explored more fully in another section.
  2. To do this conversion, remember that the number of cubic metres in a cubic kilometre is 1000 × 1000 × 1000 or one billion, 109 m3. So, to convert from cubic kilometres per year to metres per second we must multiply by 109 and then divide by the number of seconds in a year, about 31.5 million.
  3. A note on the term "watershed": In British usage the term "watershed" means "drainage divide", typically a ridge from which water is "shed" to either side. However, in North America "watershed" is commonly used to describe a whole drainage basin, an area bounded by drainage divides. Because of this ambiguity we avoid the term "watershed" in the main text.
  4. Strictly speaking, we should say "high power" because we are talking about the rate at which potential energy of the water is converted into mechanical energy of movement. However, the term "high energy environment" is most used in the study of sedimentation.
  5. Many of these processes are termed "landslides", but this term is used in several different senses. For some, it can include almost any mass wasting process, but others include only those processes where a distinct slide surface is present. Because of this ambiguity we avoid the term "landslide" here, but we do follow the general classification of mass wasting phenomena laid out in Highland & Bobrowsky (2008) "The Landslide Handbook" US Geological Survey Circular 1325)
  6. The name "Frank Slide" is used for this event and its deposit, for example in local museums, memorials, and guidebooks, but by most definitions it was not a slide but a flow.
definition

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