Geology Lab: Streams and Rivers

Features Associated with Steam Channels

As stream channels evolve, characteristic features will develop. Generally speaking, features associated with the upper reaches of a stream channel are the result of higher gradients and lower sinuosity. Many of these features are formed by down-cutting and headward erosion. In the lower reaches as gradient decreases, sinuosity increases and lateral erosion will eventually develop hairpin loops known as meanders.

Drainage patterns

Drainage patterns form as a result of stream valleys intersecting. The drainage pattern is strongly influenced by the rock types that underlie the area. If the rock is homogeneous, the pattern tends to resemble a tree in shape (dendritic), while an area of linear ridges and valleys develop a pattern in which streams mostly follow the valleys, but can cut through the ridges (trellis). Streams may radiate away from a central point (radial), or they can follow regularly fractured rock (rectangular).

Stream's valley

The area occupied by water in a stream is the stream's channel. A stream's valley is the region directly and indirectly eroded by the stream, and it extends well beyond the stream channel. A natural levee is a higher bank next to a stream where the stream has deposited sediment as it overflows or floods. The U.S. Army Corps of Engineers have installed artificial levees along major waterways to protect valuable infrastructure. The geographic area drained by a stream is referred to as the drainage basin and the boundary that separates one drainage basin from another is known as a drainage divide.

Tributaries

In regions downstream from the headwaters, the gradient or slope of a stream decreases while its discharge increases because of the additional water being added by tributaries. As the level of the channel begins to approach base level, the stream's energy is directed from side to side and the channel begins to follow a more sinuous path, or meanders. Lateral erosion by the meandering stream widens the valley floor and a floodplain begins to form. Along the inside of a meander, water velocity is less and allows for sand and gravel to be deposited in point bar deposits. On the outside of the meander, the outer bank is called the cut bank, because water velocity and turbulence are greater and the abrasive energy of the water and suspended sediments erodes the stream bank.

An Introduction to Streams and Rivers

Of all the agents that shape the Earth's surface, running water is the most important. Streams and rivers are responsible for producing a vast array of erosional and depositional landforms in both humid and arid regions. Many terms are used to describe streams, such as creeks, brooks, rills, rivers; but all refer to a channelized body of water that flows across the landscape in response to gravity. The water within these channels carries sediment from an erosional environment to a new location and eventually deposits this material. These deposits are called fluvial deposits, meaning "having to do with streams or rivers."

Features of Early Landscape Stage

Early: Erosion Processes - 1. Down-cutting is the fluvial erosion and abrasion that occurs in the bed of the stream to deepen the channel. 2. Stream has a steep gradient and carries the maximum size sediment. 3. Mass wasting of the valley walls in early stages creates a steep V-shaped valley. 4. Headward erosion lengthens the stream valley at the headwaters. Resultant stream characteristics and landforms - 1. Low sinuosity 2. Steep gradient 3. Rapids and waterfalls 4. Narrow V-shaped valleys 5. Plateaus between stream branches make wide, flat divides. 6. Drainage of area incomplete - fewer streams per unit area.

Features of Middle Landscape Stage

Erosion processes: 1. Down-cutting continues but at a lesser rate. 2. Slope retreat widens the stream valley. 3. Mass wasting of the valley walls continues with processes such as soil creep and landslides. 4. Reduction of divides and widening of valleys replace headward erosion as a dominant process. Resultant Stream Characteristics and landforms: 1. Low to moderate sinuosity 2. Floodplains begin to develop 3. Moderate gradient 4. Valleys still retain V-shape, the V becomes wider with a slight rounding of the channel. 5. Rounded to sharp divides, areas between tributaries are mountainous or hilly, not plateaus. 6. Maximum relief with fewer flat horizontal land areas.

Features of Late Landscape Stage

Erosion processes: 1. Sedimentation occurs more frequently; stream loses its energy due to a decrease in gradient. 2. Fluvial deposition occurs in point bars and floodplains. 3. Point bars are without vegetation on the inside of meander bends. 4. Lateral erosion and meandering develops. Cut banks form on the outside of meander bends. 5. Flooding and deposition of fine grained sediments occurs in floodplains and natural levees develop. Resultant stream characteristics and landforms: 1. High sinuosity 2. Low gradient which allows meandering. 3. Meanders and point bars develop. Stream may be braided with mid-channel sand bars. 4. Ox-bow lakes, cut-off meanders form. 5. Floodplain is well-developed, drainage decreases as land flattens and swamps form. 6. Monadnocks - isolated hills of more resistant rock remain. 7. Divides become nearly flat, close to stream elevation with low relief.

Features of Rejuvenation Landscape Stage

Erosion processes: 1. Uplift of a late stage meandering stream occurs. 2. Downcutting and entrenchment of meanders is the primary active erosional process, resulting from a relative drop in base level. 3. Lateral erosion is abandoned. 4. Initial stages of mass wasting creates steep V-shaped valleys. Resultant stream characteristics and landforms: 1. High sinuosity inherited from the stream when it was in late stage. 2. Steep gradient - high sinuosity and steep gradient is an unusual combination. 3. Entrenched or incised meanders develop as the stream follows the path of late stage stream. 4. Terraces form along the steep bank as down-cutting continues. 5. Steep V-shaped valleys or canyons, the stream's meanders no longer migrate laterally. 6. Plateaus - wide flat divides.

Principles that control streams

1. Gravity causes water in streams to flow down slope. The motion of the water is a kind of energy (kinetic energy) that causes the stream to erode rock, transport and deposit sediment, and modify the shape of its valley. 2. The amount of energy of the water in the stream, and therefore the amount of work it does, depends on its volume and velocity. 3. The stream's kinetic energy can transport sediment by rolling, sliding, or hopping particles along the bottom (bed load), and by suspension in the water (suspended load). The chemical properties of water also permit it to carry sediment in solution (dissolved load). 4. When a stream's velocity or its volume increases, its ability to carry sediment increases, and it will erode material from its bottom and sides. The most dramatic changes in a stream's channel shape occur during flooding. 5. When a stream slows, or loses water by infiltration or evaporation, its capacity to carry sediment diminishes, and fluvial deposition occurs.

Stream or River

A stream or river is any body of water that flows in a natural channel; people who study natural water systems and the transfer of surface water from one body to the next are referred to as hydrologists. Streams are the most important geologic agent on the Earth's surface; they may vary in size and appearance, but the principles that control them are the same.

Partial List of Major Rivers Ranked by Discharge

Hydrologists base streams on their discharge rate. Although the Nile River is the longest river in the world, it ranks #92 based on discharge. The Amazon river, ranked #1, has a considerably larger drainage area, and a much greater discharge. Amazon (1) - 6453 km, 6915000 km^2, 209000 m^3/s, Atlantic Ocean. Congo (2) - 4371 km, 4014500 km^2, 41200 m^3/s, Atlantic Ocean. Yangtze (7) - 6418 km, 1808500 km^2, 30166 m^3/s, East China Sea. Mississippi-Missouri (15) - 6270 km, 2980000 km^2, 16792 m^3/s, Gulf of Mexico. Mekong (18) - 4023 km, 811000 km^2, 14800 m^3/s, South China Sea. Saint Lawrence (28) - 3058 km, 1030000 km^2, 10100 m^3/s, Gulf of Saint Lawrence. Columbia (39) - 2000 km, 668000 km^2, 7500 m^3/s, Pacific Ocean. Yukon (47) - 3187 km, 854696 km^2, 6428 m^3/s, Bering Sea. Nile (92) - 6650 km, 3400000 km^2, 2830 m^3/s, Mediterranean Sea.

Discharge rate

Hydrologists typically quantify or rank streams by their discharge rate, defined as the total volume of water passing a point in a particular period of time. The water level in a stream is called its stage, and historical stage levels and discharge rates are often used to estimate the probability of flooding in a particular area. The character of flooding depends on various stream channel features. It can be confined but fast in narrow valleys upstream near the headwaters or source; or it can be widespread but rising slowly in downstream areas where the slope of the stream is low and its path winding in the floodplain nearer the mouth or base level.

Tributaries Figures

In the lower reaches of a stream channel, once the gradient has significantly decreased, the stream will meander to diffuse excess energy. Erosion is increased along the outside of these meanders, while deposition occurs on the inside of the bends. Lateral erosion will continue to increase sinuosity. Along the A - A' profile, the difference in the stream banks in a meander bed shows the over-steepened slope of the cut bank, and the gentle slope of the point bar deposits. As erosion continues, the cut bank will retreat into the bedrock, continuing to increase sinuosity. Deposition will continue, thus the meanders will migrate over time.

Sinuosity Index Values for Rivers and Streams

Sinuosity index values are derived by measuring the length of the stream's channel over a specified distance, divided by the straight line distance. <1.05 - almost straight 1.05<=SI<1.25 - winding 1.25<=SI<1.50 - twisty 1.50<=SI - meandering

Sinuosity

Sinuosity, or the sinuosity index, is a measure of how winding the course of a stream is. To determine the sinuosity index, measure the length of the channel along the stream's path and divide by the straight-line distance down the valley. Unlike gradient, both measurements must be in the same unit. For rivers, the most conventional classes of sinuosity are listed below.

Drainage Pattern Figure

Stream drainage patterns are a result of the interaction of the stream channel on the underlying bedrock. Dendritic drainage patterns develop on flat-lying rocks. Trellis drainage patterns develop in eroded mountainous regions where down-cutting by major rivers have cut across ridges. Radial drainage occurs when water drains off a high standing feature such as a volcano or mountains. Rectangular drainage develops when water flows across fractured rock.

Stream Valleys

Stream valleys are the most common landforms on the Earth's surface. In their upper reaches, near the headwaters, rivers typically have steep slopes and V-shaped valleys, while downstream the valleys decease in steepness and become wider. The V-shaped part is referred to as "youthful," while downstream the valley is said to become more "mature." These terms refer to specific characteristics that control stream morphology, not the actual age of the valley.

Stream Erosion and Landscape Evolution

Streams and stream valleys evolve through three stages, commonly referred to as early, middle, and late stage. (Some literature will refer to these stages as youthful, mature, and old age). Streams are very effective at shaping the landscape and cause more erosion that any other natural or human process. Streams are always working; eroding rock and soil, transporting sediment, and depositing this material in a new environment. The features we see on the Earth's surface today are just a snapshot of current conditions; and although geologic change takes place very slowly, sediment everywhere is moving and changing the landscape thanks to rivers, streams, waves, and current activity.

Rejuvenation

Streams can carve through uplands, forming canyons in the early stage; widen those canyons into broad valleys in a hilly terrain in the middle stage; and then reduce the hills to a broad plain in the late stages of landscape evolution. If the broad plain undergoes uplift, a fourth distinctive stage of landscape evolution can occur, called rejuvenation.

Stream Gradient

The changes in a stream along its course are primarily due to the change in the gradient. Because gravity is the driving force for stream flow, the steepness of the slope of a stream, known as the stream gradient, is important in determining certain characteristics of the stream. Stream gradient is calculated as the drop in elevation divided by the horizontal distance the stream travels. The gradient can be determined for the whole stream, the average gradient, or for a segment of the stream using the following formula: Stream gradient = change in elevation / horizontal distance. Gradient is generally expressed in feet per mile or meter per kilometer, depending on the scale of the map. The gradient near the headwaters of a stream is steep, and diminishes gradually to very low values in the floodplain and delta near the mouth.

Landscape Evolution

As landscapes evolve through stream erosion processes, distinctive features will develop. Hawaiian waterfalls, rapids and straight stream segments are characteristic found in an early landscape stage. Wider channels, lower gradient streams, and developed floodplains are characteristic of middle landscape stages seen in the Arkansas River Valley. Developed meanders, ox-box lakes, and wide floodplains are characteristics of later landscape stages as seen in the Mississippi River Valley. Entrenched meanders and steep walled canyons are characteristic of rejuvenated landscapes, as seen across the Colorado Plateau; the Colorado River is incising through layers of sedimentary rock near Maob, Utah.

Three Principle Mechanisms of Stream Erosion

As stream channels transfer their kinetic energy from the water to rocks over which they flow, streams erode and alter the channel and stream valley. The three principle mechanisms of stream erosion are: Headward erosion, which extends the head of the valley upslope Downcutting, which deepens the channel toward base level, Lateral erosion, which widens the valley walls. These erosive forces modify the stream channel and surrounding stream valley by cutting into the rocks and redistributing sediment. The sediment carried by the stream, including the bed load, suspended load, and dissolved load, also work in concert with the water to modify the channel.

Chute

Near the mouth of a stream where the channel is nearly at base level, maximum discharge occurs and meandering often becomes very pronounced. Widespread lateral erosion by the meandering stream produces a floodplain that is often several times wider than the river's meander belt. When meanders become so sinuous that the thin area of land between two meanders is cut through, a chute is formed. The cut-off meander forms a crescent shaped lake, known as an oxbow lake, which may eventually be filled with fine grained sediment and organic matter. This meander scar is easy to identify on a topographic map due to the curvature of contour lines. Periodically, streams may receive more water than they are capable of carrying. If this occurs, flood waters will spill over their banks, leaving behind a ridge of silty sediments. These natural levees run parallel to the stream channel and form as a result of successive river flooding. After flood waters spill over the banks, they spread across flat, planar floodplains adjacent to the channels. During waning stages of a flood, velocity decreases and suspended silt and clay settle out over the floodplain.

Rapids and Waterfalls

Often, the head of a stream, or the source area, is well above base level. At the headwaters, streams typically have steep slopes and down-cutting prevails. As the stream deepens its valley, it may encounter rocks that are resistant to erosion and form rapids and waterfalls. In arid areas, streams often erode narrow valleys with nearly vertical walls. In humid areas, the effect of mass wasting and slope erosion caused by precipitation typically produces V-shaped valleys.