STREAMS
Hydrologic Cycle: All the water there is is all the water there is.....
Water is held and recycled in the Earth System. Water evaporates off the surface and transpires from vegetation. Precipitation returns water to the surface. Water on the surface infiltrates or runs off. Water runs off the surface in streams.
Stream: A body of water confined to a channel. Geologists refer to all channelized water as "stream" regardless of size.
Stream terms:
Head
Mouth
Channel
Bed
Banks
The Longitudinal profile depicts the shape of a stream from head to mouth.
Floodplain: area adjacent to the stream
Gradient: slope of the stream, determines the velocity of the stream.
Drainage basin: the area drained by an individual stream
Discharge: the volume of water that flows past a given point for a period of time. (ft/sec)
Divide: the higher topography that separates one drainage basin from another.
Tributary: smaller stream that feeds a larger stream.
Streams alter a landscape in three ways:
Streams act to erode a landscape downward and laterally. Streams transport eroded material. Streams deposit material.
Erosion:
Base Level: the level to which a stream can erode a landscape. Ultimate base level is sea level.
As base level changes the profile of the stream changes.
If base level raises then the stream slows and deposition is the result. If base level is lower then stream erosion is increased. Base level changes in response to lowering and rising sea level, building of a dam across a stream channel or regional uplift of the area over which a stream flows.
As a stream flows across a landscape its velocity is dictated primarily by gradient. The maximum velocity within the stream itself varies along the stream's length due to the shape and roughness of the channel, its banks and bed. In general velocity will be highest in the center of a straight stream section and highest on the outside of a curve in a curved section of stream. A stream with a bed that is rough with boulders will be slower than a stream with a smooth bed.
High gradient: also called a young stream
In streams with a steep gradient (think mountain stream) the erosion action is predominantly downcutting. The stream is narrow with few meanders (curves) and erodes downward. Waterfalls are common. The stream bed has mostly large particles because the smaller particles have been carried away by the high velocity of the water. The stream has a "V" shaped* cross section. There is little or no floodplain.
Low gradient: also called mature stream
As a stream approaches base level the gradient decreases and the velocity slows. Erosion is predominantly side to side forming a broad flat floodplain. Erosion acts to carve a large floodplain across the landscape. There are many meanders and the stream is wide with a "U" shaped cross section. Oxbows are common. The stream bed is composed of smaller particles because the velocity is lower.
Streams act to carve a stream valley in a landscape as described above. They not only carve downward and laterally but also erode a landscape headward. Headward erosion lengthens a a stream in a headward direction through mass wasting, gullying and sheet erosion. Sometimes streams will intersect and stream piracy occurs, changing the flow of the captured stream.
Erosion within the stream channel itself occurs in a variety of ways.
Hydraulic Action: loosens material from bed and banks releasing it into the water for transport.
Solution: a slow process of dissolving rock material over which the stream flows
Abrasion: Friction and impact on the stream bed and banks by material carried by the stream. Potholes form when swirling water erodes depressions into the bedrock of the stream bed and banks.
Transport:
Load is the term used to describe the material carried by a stream.
Bed Load: Material that moves along the bed of a stream. Transported by sliding, rolling and saltation (bouncing). Usually sands and larger.
Suspended Load*: Material that is suspended in the water column. Material is transported with the water as it flows. Silts and clays.
Dissolved load: Material dissolved in the water. Chemical ions
The Mississippi River transports 450 million tons of sediment to the Gulf of Mexico each year.
Deposition:
Whenever water velocity slows deposition of the load occurs. Deposition includes:
Meanders: (both erosional and depositional) water flows in a curved path, Deposition occurs on the inside curve of a meander (point bar*) where velocity is slower, erosion occurs on the outside of a meander where velocity is higher. This acts to enlarge and exaggerate* the meander eventually forming a meander cutoff or oxbow lake. Through meander growth and migration a stream moves laterally across a floodplain.
Delta: When a stream flows into another body of water and its velocity is suddenly slowed sudden deposition occurs. This deposition is in a fan shape grading outward from coarse to fine. Deltas will grow outward* as well as migrate laterally over time. Map of Mississippi Delta*
Floodplain Deposits:
Floodplain*: the flat area adjacent to a stream. During periods of high water flow (spring snow melt and heavy rains) the stream overflows its banks onto the floodplain*. That's why its called a FLOOD plain.
Floods classified as yearly, ten year, 50 year ect based on probability of occurrence.
As water flows over the banks of a stream several deposits are laid down. In general the coarsest material drops out first grading to finer.
Natural Levee: Deposit along the bank that builds up over time. Composed of the coarsest components. Picture of natural levees*, the levees are the high linear features sticking up above the flood waters.
Floodplain sediment: Material grading with finest outward of sediment. Sediment is composed of suspended load from stream. This suspended material is what makes floodplain soils such good agricultural soil.
Floods:
Stream flow is a balance between runoff and infiltration. If infiltration decreases more runoff results. Stream flow varies with the season.
Average flow refers to the average amount of water discharged by season.
Stage: the elevation of the water in a stream.
Flood stage: the elevation at which the water will overflow its banks.
Crest: the highest elevation a stream reaches during a flood event.
Factors that enhance flooding:
- Topography: steeper slopes result in more runoff and less infiltration.
- Vegetation: vegetation acts to trap water, slowing runoff and encouraging infiltration.
- Ground saturation: in large weather events if the ground becomes saturated infiltration is impossible.
- Urbanization: if an area is paved over or covered by a building then infiltration is reduced.
Upstream Floods: usually occur in steeper areas, Flash floods, these floods are rapid events that drain quickly. Localized
Downstream Floods: occur in areas of flatter topography. Slow events that are widespread and take a long time to drain.
Case Study: Mississippi Floods of 1993
Flood Controls:
- Levees: levees act to raise the banks of a stream. Depending on the construction techniques they can be very expensive. See case study.
- Dams: not always feasible due to topography.
- Channelize flow: this helps alleviate flooding in the area of channelization but tends to move the high water downstream.
- Retention Ponds: these act to trap water and keep runoff to a minimum, enhancing infiltration.
- Diversion Channels: during flood events water is often diverted away from populated areas through diversion channels. This floods someone else.
- Move: relocate off of the floodplain
There are also ways to engineer structures that allow a flood to occur that will not harm the structure.
