10.6: Surface Water

A stream or river is a body of flowing surface water confined to a channel. Terms such as creeks and brooks are social terms not used in geology. Streams are the most important agents of erosion and transportation of sediments on the earth’s surface. They create much of the surface topography and are an important water resource. Most of this section will focus on stream location, processes, landforms, and flood hazards. Water resources and groundwater processes will be discussed in later sections.

Discharge

Several factors cause streams to erode and transport sediment, but the two main factors are stream channel gradient and velocity. The stream gradient is the slope of the river channel. A steeper gradient promotes downward stream erosion. When tectonic forces lift up a mountain, the increased stream gradient causes the stream to erode downward and make a valley. Stream velocity is the speed of the flowing water in the channel. Velocity can increase by increasing the gradient, decreasing cross-sectional area (narrowing) of the channel (reducing friction), or by increasing the discharge.

Stream size is measured in terms of discharge, i.e. the volume of water flowing past a point in the stream over a defined time interval. Smaller streams have a smaller discharge, therefore generally stream discharge increase downstream. Volume is commonly measured in cubic feet (length x width x depth), shown as feet³ or ft³. Therefore, the units of discharge are cubic feet per second (ft³/sec or cfs). For example, the Mississippi River is the largest river in North America, with an average flow of about 600,000 cfs [19]. For comparison, the average discharge for the Jordan River at Utah Lake is about 574 cfs [20] and for the Amazon River (the world’s largest river), annual discharge is about 6,200,000 cfs [21].

When the channel narrows but discharge remains constant, the same volume of water flows through a narrower space causing the velocity to increase, similar to putting a thumb over the end of a backyard water hose. In addition, during rainstorms or heavy snowmelt, runoffs will increase which increases stream discharge and thus velocity.

Velocity varies within the stream channel as well. Generally, when the channel is straight and uniform in-depth, the highest velocity is in the center of the channel along the top of the water where it is the farthest from frictional contact with the channel bottom and sides. When the channel curves, the highest velocity will be on the outside of the bend as it has to flow faster to keep up!

Figure \(\PageIndex{1}\): In a river bend, the fastest moving particles are on the outside of the bend, causing erosion. The slowest are on the inside bend, causing deposition. In section A, water flows fastest in the center (longer arrows indicate faster velocity). In section B, water flows fastest on the outside bend (Earle 2015).

Runoff vs. Infiltration

There are many factors dictating whether water will infiltrate into the ground or runoff over the land after precipitation. These include but are not limited to the amount, type, and intensity of precipitation, the type and amount of vegetative cover, the slope of the land, the temperature and aspect of the land, preexisting conditions, and the type of soil in the area of infiltration. High-intensity precipitation as rain will cause more runoff than the same amount of rain spread out over a longer duration. If the rain falls faster than the properties of the soil allow it to infiltrate, then the water that cannot infiltrate becomes runoff. Dense vegetation can increase infiltration, as the vegetative cover slows the overland flow of water particles, giving them more time to infiltrate. If a parcel of land has more direct solar radiation and/or higher seasonal temperatures, there will likely be less infiltration and runoff, as evapotranspiration rates will be higher. As the slope of the land increases, so does runoff, as the water is more inclined to move downslope than infiltrate into the ground. Extreme examples are a basin and a cliff, where water infiltrates much quicker into a basin than a cliff having the same soil properties. Because saturated soil does not have the capacity to take more water, runoff is generally greater over-saturated soil. Clay rich soil cannot accept infiltration as quickly as gravel-rich soil.

Drainage Patterns

The pattern of tributaries within a region is called a drainage pattern. They depend largely on the type of rock beneath, and on structures within that rock (such as folds and faults). The main types of drainage patterns are dendritic, trellis, rectangular, radial, and deranged. Dendritic patterns are the most common and develop in areas where the underlying rock or sediments are uniform in character, mostly flat-lying, and can be eroded equally easily in all directions. Examples are alluvial sediments or flat-lying sedimentary rocks. Trellis patterns typically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Appalachian Mountains in the eastern United States have many good examples of trellis drainage. Rectangular patterns develop in areas that have very little topography and a system of bedding planes, joints, or faults that form a rectangular network. A radial pattern forms when streams flow away from a central high point such as a mountain top or volcano, with the individual streams typically having dendritic drainage patterns. In places with extensive limestone deposits, streams can disappear into the groundwater via caves and subterranean drainage and this creates a deranged pattern.

Figure \(\PageIndex{1}\): Various stream drainage patterns.

Fluvial Processes

Fluvial processes are the mechanisms that dictate how a stream functions and include factors controlling fluvial sediment production, transport, and deposition. Fluvial processes include velocity, slope and gradient, erosion, transportation, deposition, stream equilibrium, and base level.

Streams can be divided into three main sections: the many smaller tributaries in the source area, the main trunk stream in the floodplain and the distributaries at the mouth of the stream. These can be defined as zones of sediment production (erosion), transport, and deposition.

Zone of Sediment Production (Erosion)

The zone of sediment production is located in the headwaters (the start) of the stream were very small channels erode sediment and contribute to larger tributary streams. These tributaries carry sediment and water further downstream to the main trunk of the stream. Tributaries at the headwaters have the steepest gradient and most sediment production and erosion. Headwater streams tend to be narrow and straight. Since the zone of sediment production is generally the steepest part of the stream, many headwaters are located in relatively high elevations. For example, the Rocky Mountains of Wyoming and Colorado contain much of the headwaters for the Colorado River which then flows from Colorado through Utah, Arizona, to Mexico.

Zone of Sediment Transfer (Transportation)

Downstream of the headwaters, the stream erodes less sediment but transports the sediment provided from the headwaters in the zone of sediment transfer.

Figure \(\PageIndex{1}\): A stream carries dissolved load, suspended load, and bedload.

Sediment transportation is directly related to stream gradient and velocity. Faster and steeper streams can transport larger sediment grains. When velocity slows down, larger sediments settle to the channel bottom. When the velocity increases, those larger sediments are entrained and move again.

Transported sediments are grouped into bedload, suspended load, and dissolved load. Sediments moved along the channel bed are the bedload and typically are the largest and densest. Bedload is moved by saltation (bouncing) and traction (being pushed or rolled along by the force of the flow). When stream velocity increases, smaller bedload sediments can be picked up by flowing water and held in suspension as suspended load. The faster streams can carry larger grains as suspended load. Dissolved load in a stream is the sum of the ions in solution from chemical weathering. The dissolved load includes ions such as bicarbonate (HCO₃^-), calcium (Ca²⁺), chloride (Cl^-), potassium (K⁺), and sodium (Na⁺).

Figure \(\PageIndex{1}\): Profile of stream channel at bank-full stage, flood stage, and deposition of natural levee

Stream flooding is a natural process that adds sediment to floodplains. A floodplain is the generally flat area of land located adjacent to a stream channel that is inundated with floodwater on a regular basis. As soon as the flooding stream overtops its banks and occupies the wide area of its floodplain, the velocity decreases. Sediments are added to the floodplain during this flooding process.

Zone of Disposition

The process of deposition occurs when bedload and suspended load come to rest on the bottom of the water column in a stream channel, lake, or ocean. The two major factors causing deposition are the decrease in stream gradient and the reduction in velocity. These can be associated with a decrease in discharge or increased in cross-sectional area. Deposition occurs temporarily in the zone of transportation such as along meandering stream point bars, floodplains, and alluvial fans (discussed later), however, ultimate deposition occurs at the mouth (the end) of the stream where it reaches a lake or ocean. These deposits at the mouth of a stream form landforms called deltas. Deposition at the mouth of a stream is generally of the finest sediment such as fine sand, silt, and clay, because as the stream exits its channel, the energy of the water is completely dispersed, causing the deposition of all particles in the stream.

Landforms

Stream landforms are the land features formed on the surface by either erosion or deposition. The primarily stream-related landforms described here are related to channel types.

Channel Types

Figure \(\PageIndex{1}\): Braided stream pattern on the Waimakariri River in New Zealand.

Figure \(\PageIndex{1}\): Air photo of the meandering river, Río Cauto, Cuba.

Stream channels can be straight, braided, meandering, or entrenched. The gradient, sediment load, discharge, and location of the base level all influence channel type. Straight channels are relatively straight, located near the headwaters, have steep gradients, low discharge, and narrow V-shaped valleys. Good examples of these are located in mountainous areas.

Braided channels have multiple smaller channels splitting and recombining downstream creating numerous mid-channel bars. These are found in broad terrain with low gradients near sediment source areas such as mountains or in front of glaciers, for example in Alaska.

Meandering channels are composed of a single channel that curves back and forth like a snake within its floodplain. Meandering channels tend to have a wide floodplain, high discharge, natural levees, and flood regularly. Meandering channels are usually located on low gradient slopes where the stream emerges from its headwaters into the zone of transportation and extends close to the zone of deposition at the stream’s mouth. In areas of uplift, like has occurred on the Colorado Plateau, meanders that formed on the upland can become entrenched or incised as the stream cuts its meandering pattern down into bedrock.

Entrenched channels occur when a meandering channel rapidly down cuts due to a drop in base level. This causes the original meandering shape to be preserved within a deeply entrenched channel. This channel type is rare but an example of this process is where the Colorado River and other streams crossed the Colorado Plateau as meandering streams. As the Colorado Plateau has uplifted over the past several million years, the Colorado River has incised into the flat-lying rocks of the plateau by hundreds of feet.

Figure \(\PageIndex{1}\): Entrenched meander of the Colorado River, downstream of Page, Arizona.

Figure \(\PageIndex{1}\): The San Juan River has incised meanders into the flat Colorado Plateau at Goosenecks State Park, southeastern Utah.

Alluvial Fans

Figure \(\PageIndex{1}\): Alluvial fan in Iraq seen by a NASA satellite. A stream emerges from the canyon and creates this cone-shaped deposit.

Alluvial fans are a depositional landform created where streams emerge from mountain canyons into a valley. The channel that had been confined by the canyon walls and suddenly is no longer confined slows down and spreads out, dropping its bedload of all sizes. It is like a delta above water. As distributary channels fill with sediment, the stream is diverted laterally, and the alluvial fan develops into a cone shape with distributaries radiating from the canyon mouth. Alluvial fans are common in the dry climates of the West where ephemeral streams (streams that run periodically) emerge from canyons in the ranges of the Basin and Range.

Floodplains, Meandering Levels, and Natural Levees

Figure \(\PageIndex{1}\): Landsat image of lower Mississippi River floodplain

Many fluvial landforms occur in a floodplain near a meandering stream. A stream creates its floodplain as the channel meanders back and forth over thousands, even millions of years. Regular flooding contributes to creating the floodplain by eroding uplands next to the floodplain.

The location and width of floodplains naturally vary, however, humans build artificial levees on flood plains to limit flooding. Floodplains are nutrient-rich from the fine-grained deposits and thus often make good farmland. Floodplains are also easy to build on due to their flat nature, however, when floodwaters crest over human-made levees, the levees quickly erode with potentially catastrophic impacts. Because of the good soils, farmers regularly return after floods and rebuild year after year.

Figure \(\PageIndex{1}\): Point bar and cut bank on the Cirque de la Madeleine in France.

Meandering rivers create additional landforms as the channel migrates within the floodplain. Meandering rivers erode side-to-side because the highest velocity water having the most capacity to erode is located on the outside of the bend. Erosion of the outside of the bend of a stream channel is called a cut bank. Opposite the cut bank on the inside bend of the channel is the lowest stream velocity and therefore becomes an area of deposition call a point bar.

Figure \(\PageIndex{1}\): Meander nearing cutoff on the Nowitna River in Alaska

Through erosion on the outsides of the meanders and deposition on the insides, the channels of meandering streams move back and forth across their floodplain over time. Sometimes on very broad floodplains with very low gradients, the meander bends can become so extreme that they cut across themselves at a narrow neck. The former channel becomes isolated from streamflow and forms an oxbow lake.

Figure \(\PageIndex{1}\): Meander nearing cutoff on the Nowitna River in Alaska

Deltas

Figure \(\PageIndex{1}\): Location of the Mississippi River drainage basin and Mississippi River delta.

When a stream reaches a low energy body of water such as a lake or some parts of the ocean, the velocity slows and the bedload and suspended load sediment come to rest, forming a delta. If wave erosion from the water body is greater than deposition from the river, the deposition will not occur and a delta will not form. The largest and most famous delta in the United States is the Mississippi River delta formed where the Mississippi River flows into the Gulf of Mexico. The Mississippi River drainage basin is the largest in North America, draining 41% of the contiguous U.S. [24]. Because of the large drainage area, the river carries a large amount of sediment that is supplied to the delta. The Mississippi River is a major shipping route and human engineering has ensured that the channel no longer meanders significantly within the floodplain. In addition, the river has been artificially straightened so that it meanders less and is now 229 km shorter than it was before humans began engineering it [24]. Because of these restraints, the delta is now solely focused in one area and thus has created a “bird’s foot” pattern. These two NASA images of the delta (follow the link) show how the shoreline has retreated and the land was inundated with water while deposition of sediment was located at end of the delta. These images have changed over a 25 year period from 1976 to 2001. These are stark changes illustrating sea level rise and land subsidence from the compaction of peat due to the lack of sediment resupply [25].

Figure \(\PageIndex{1}\): Before (left) vs. After (right)

Deltas represent stream deposits protruding into a quiet water body and can be further categorized as wave-dominated or tide-dominated. Wave-dominated deltas occur where the tides are small and wave energy dominates. An example is the Nile River delta in the Mediterranean Sea that has the classic shape like the Greek character (Δ) from which the landform is named. A tide-dominated delta is when ocean tides are powerful and influence the shape of the delta. For example, Ganges-Brahmaputra Delta in the Bay of Bengal (near India and Bangladesh) is the world’s largest delta and mangrove swamp called the Sundarban.

Sundarban Delta in Bangladesh, a tide-dominated delta of the Ganges River Tidal forces creates linear segments in the delta shoreline by ocean intrusion into the delta deposits. This delta also holds the world’s largest mangrove swamp, and incidentally is the only place where the Bengal tiger still actively hunts humans as prey.

Figure \(\PageIndex{1}\): Nile Delta showing its classic “delta” shape.

Terraces

Stream terraces are remnants of older floodplains located above the existing floodplain and river. Like entrenched meanders, stream terraces form when uplift occurs or base level drops and streams erode downward, leaving behind their old floodplains. In other cases, stream terraces can form from extreme flood events associated with retreating glaciers.

Figure \(\PageIndex{1}\): Terraces along the Snake River, Wyoming.

Figure \(\PageIndex{1}\): Terraces in Glen Roy, Scotland

References

20. Cirrus Ecological Solutions (2009) Jordan River TMDL. Utah State Division of Water Quality

21. United States Geological Survey (1967) The Amazon: Measuring a Mighty River. United States Geological Survey

24. Turner RE, Rabalais NN (1991) Changes in Mississippi River water quality this century. Bioscience 41:140–147

25. Törnqvist TE, Wallace DJ, Storms JEA, et al (2008) Mississippi Delta subsidence primarily caused by compaction of Holocene strata. Nat Geosci 1:173–176