Geol 33 Environmental Geomorphology
J Bret Bennington
Streams
Streams are flowing surface water of two origins: overland flow (direct runoff from rain or snowmelt) and baseflow (groundwater discharging into the stream from below).
Perennial streams - have water flowing through them most or all of the time. Perennial streams are characteristic of humid regions and tend to be gaining streams, meaning that they are fed by baseflow. Perennial streams tend to increase in discharge (amount of water moving through the stream channel) over distance.
Ephemeral streams - are characteristic of arid regions and are usually dry, only flowing during the wet season or after episodic storms. Ephemeral streams tend to be losing streams, meaning that their discharge decreases over distance as water seeps through the channel into the water table.
Perennial streams may also become losing streams temporarily during floods. If the surface of the stream is elevated above the water table then water will flow from the stream into the bank sediments (bank storage). This water will later be released back into the stream as baseflow after the stream level begins to fall.
The slope down which the stream flows is called the stream gradient. A stream tends to flow along the shortest path of least resistence in a well defined channel. It continues to flow until it reaches an area of standing water such as a lake or a sea.
The elevation of the standing water is called the base level of the stream. Base level is the level below which the stream does not flow and so cannot erode. When one speaks of base level, one is usually referring to sea level, a sort of universal base level. Other base levels, such as those associated with lakes or closed basins on land, are called local base levels.
The amount of water flowing past a point along the stream over a given interval of time is called the streamâs discharge. In general, discharge increases downstream as tributaries empty into and add their discharge to the stream.
The relationship of discharge at any point along the stream to its channel and its velocity is given by the equation:
Q (discharge) = A (cross sectional area) x V (velocity)
As discharge increases or decreases, A and / or V must also increase.or decrease. Likewise, at constant Q, an increase in A must result in a decrease in V and visa versa. This explains the presence in a river of slow moving deep stretches interspersed with rapid flowing shallow stretches (rapids).
Also moving with the water are the sediments being transported by the stream, its load.
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The streamâs load consists of several components.
Bed load
Suspended load
Dissolved load
The amount of load that a stream can carry is related primarily to the amount of water flowing through it (discharge) and the velocity of the flow. This is often called the carrying capacity of the stream.
The size of sediment that the stream can carry is related to the velocity of its flow.
The size, shape, velocity, and other characteristics of a stream depend on all of the previously mentioned factors and their interactions. Consequently, there are different kinds of streams, and streams can change radically if their discharge changes, as during a dry spell or a flood.
Streams that are carrying a load of sediment below their carrying capacity tend to erode their channels and flow along relatively straight courses defined by major features in the landscape.
Streams that are carrying a load of sediment at or near their carrying capacity tend to erode and deposit sediment in equilibrium and flow in curved meandering paths within wide, eroded areas called floodplains.
Streams choked with excess sediments tend to flow in a braided pattern as they deposit excess sediments as bars that interrupt and divide the main channel into several anastomosing smaller channels.
Streams change their form and load as they flow from mountains to the sea in response to changes in their gradient. Thus, a stream flowing down through the mountains across a steep gradient will erode and flow along a relatively straight path until it reaches the flatter piedmont where it will slow, widen, and deposit the coarser part of its load. Upon reaching the flat plains below the piedmont, the stream will slow, widen further, and meander to the sea.
Upon reaching the sea, a stream will slow almost to a stop, dropping most of its load. This dumping of sediment at the mouth of the river chokes the mouth with sediment and leads to a branching braided pattern of flow called a distributary system. The sediment from the river and the distributaries build out into the sea to form a river delta.
It is also common for stream discharge to vary over time as well as along the length of the streams course.
For example, many large, braided rivers are found ether at the foot of glaciers where runoff is highly seasonal or in dry areas with highly seasonal rainfall. During times of high water flow, enormous amounts of sediment are added to the river channel and carried along by the flowing water. As discharge decreases, these rivers become sediment choked and take on their braided appearence. As water flows through the small, branching channels sediment is continuously eroded and redeposited, resulting in a constant movement and shifting of the courses of the channels and their branching points over the width of the overall channel.
Meandering rivers also tend to move their channels within the larger floodplain. This process, called meandering occurs because of the tendency for curves in the river to be accentuated as the outside bank of the curve erodes due to the increase in velocity of the water travelling along the outside of the meander. Water moving along the inside slows going around the bend and drops part of its load to form a point bar. The the point bar and the outside of the meander grow, the loop becomes more extreme. Eventually, one loop may catch up with another that is slowed in its growth by more resistant banks, or a flood may cause a the river to cut a more direct channel. In either case, the abandoned loop becomes isolated from the main channel and forms an oxbow lake.
Floods
If a streamâs discharge increases to the point where the channel of the stream cannot contain its flow, the stream will overflow its banks and run out onto the floodplain. Floods are usually associated with above-average periods of rainfall or rapid snowmelt. Floods can also be caused by dam failure.
Several things happen to a river when it floods.
The increase in discharge causes the river to flow much more rapidly than usual, which greatly increases the load capacity of the river and causes it to erode its channel and carry much more and larger sediments than it normally does. Large boulders in river beds are moved during floods, and human structures such as bridge pilings and concrete barriers can be swept away by the rushing floodwaters. Most of the work performed by streams takes place during floods. As the flood subsides, the river drops its sediment load and the channel aggrades back to its previous level.
Scour - obstructions to flow in a stream channel cause local pockets of turbulence leading to rapid downward scour. Such obstructions include boulders and bridge tiers, which can be completely undermined by scour during a flood. During heavy rains in 1987 high flow on Schoharie Creek in upstate New York undermined a bridge over the NYS Thruway, causing collapse and the loss of several lives as vehicles plunged into the river.
When the river overflows its banks, it moves out onto the floodplain, effectively greatly increasing the area of its channel. This causes an immediate decrease in the velocity of the water as it leaves the channel, resulting in the deposition of sand adjacent to the river channel. This material forms a raised bank along the river called a natural levee. Finer grained silts and clays wash out with the floodwaters and are left to settle out onto the floodplain after the flood receeds. The great natural fertility of floodplain soils is a direct result of their periodic rejuvination by floodwaters bearing silt and clay.
During most floods, the floodplain acts as a temporary reservoir where the excess discharge is dumped and held until it evaporates away. This helps to alleviate flooding farther downstream. Wetlands adjacent to rivers accept and hold floodwaters, returning them to the river channel when the flood receeds.
Floods often cause the channel of the stream to straighten and change position within the flood plain. In fact, the flood plain itself can be defined as the expanse of land occupied by and eroded by the river over a long period of time and many shifts of position.
Over long periods of time, changes in base level, annual discharge, or load can cause a stream to erode rapidly downward to a new level that put the old flood plain out of reach of flooding. A new floodplain will form leaving the old floodplain standing above as a terrace.
It must be emphasized that flooding is a normal part of the long range dynamics of a river and that the floodplain is a feature that is constructed and maintained by flooding.
Equilibrium in Streams
Streams must, owing to thermodynamics, achieve over time some kind of equilibrium between discharge, load, gradient and channel characteristics. This occurs because all of these variables are interdependent and will shift until a stream is expending its potential energy as efficiently as possible.
A stream that has achieved equilibrium is said to be a graded stream. Any change in one of the important parameters will cause the stream to respond by altering other parameters to return to a graded state.
Some examples: