Channels Stream Valleys and Floodplains
Stream valleys, channels, and floodplains form complicated systems that evolve through time in response to changes in sediment supply, precipitation, land use, and rates of tectonic uplift affecting a drainage basin.
Stream channels serve to convey flow during normal periods, whereas floodplains accommodate flow above the bankfull stage (floods) that occurs with frequencies inversely proportional to their magnitude. Bankfull stage is defined as the discharge at which the water level is at the top of the channel. Any further increase in discharge will cause the water to spill out of the channel and inundate the adjacent floodplain. Flood frequency studies of streams throughout the world show a remarkably consistent 1.0 to 2.5 year recurrence interval for bankfull discharge in most streams, averaging about 1.5 years, meaning that small floods are relatively common events.
Stream channels are classified according to four basic variables: their slope or gradient (change in elevation per unit of distance along the stream channel), their width to depth ratio, their entrenchment ratio (floodplain width to bankfull width), and the predominant channel bed material (bedrock, gravel, cobble, sand, or clay). In general, the width of stream channels increases downstream more than the depth, so that large rivers such as the Mississippi may be a kilometer or more wide but only tens of meters deep. Channels with large bed loads of coarse-grained materials, steep gradients, and banks composed of easily eroded sediments tend to be shallow and braided, meaning that flow occurs through many anastomosing channels separated by bars or islands. Streams with low gradients, small bed loads, and stable banks tend to meander in space and time, following a pattern that resembles an exaggerated sine wave. Another characteristic of streams with beds coarser than sand is the occurrence of riffle-pool sequences, in which the channel is segregated into alternating deep pools and shallow riffles. In steep mountain streams, the riffles can be replaced by steep steps over boulders or bedrock outcrops to form a step-pool sequence.
Stream channels can change in form over time as a function of climate, precipitation, sediment supply, tectonic activity, and land use changes. Increased precipitation or human activities, for example, heavy grazing or clear-cut logging, can lead to increased erosion or mass wasting that subsequently increase the amount of sediment delivered to streams. As a consequence, the channel and stream gradient change to accommodate the increased sediment load, which may in term have adverse effects on aquatic habitat. For example, an influx of fine-grained sediment can clog the gravel beds in which salmon and trout spawn. The effect of urbanization is generally to increase storm runoff and the erosive power of streams because impervious areas (principally pavement and rooftops) decrease the amount of water that can infiltrate into the soil while at the same time decreasing the amount of sediment that is available for erosion before runoff enters stream channels. Tectonic uplift can increase the rate of stream valley incision. Thus, stream channels represent the continually changing response of the stream system to changing conditions through over geologic and human time spans.
Because streams are the products of continual change, many stream valleys contain one or more generations of stream terraces that represent alternating stages of sediment deposition (valley filling) and erosion (stream incision). Each flat terrace surface, or tread, is a former floodplain. Stream terraces can often be recognized by a stair-step pattern of relatively flat surfaces of increasing elevation flanking the channel; in many cases, however, stream terraces are subtle features that can be distinguished and interpreted only with difficulty.
Floodplains form an important part of a stream system and provide a mechanism to dissipate the effects of floods. When a stream exceeds bankfull discharge, floodwater will begin to spill out onto the adjacent flat areas where its depth and velocity decrease significantly, causing sediment to fall out of suspension. The construction of flood control structures such as artificial levies has allowed development on many floodplains that would otherwise be subjected to regular inundation. Artificial levies, however, also increase the severity of less frequent large floods that would have been buffered by functioning floodplains, and can thereby provide a false sense of security. Current trends in flood hazard mitigation are therefore shifting away from the construction of containment structures and towards the use of more enlightened land use practices such as the use of floodplains for parks or green belts rather than residential development.