"Bankfull"

Rivers transport most of their sediment when at "bankfull." Bankfull generally means when water just begins to flow over a streambank's inside bend. Even during a flood, most of the power to move sediment occurs within the channel area. We can predict when a river will be at "bankfull" stage (once every 1.5 to 2 years), and the probability of how frequently a river might spill into the floodplain it created and at what depths. That is why we have the terms "10-year flood; 50-year flood; 100-year flood."

In this part of the model the river is shown at "bankfull."

Since rivers will occupy the floodplain, we must give the river enough room. We cannot deny the river its desire to dissipate energy, otherwise it has even more force to gouge its banks, its bed, to rip out culverts and houses, and to undermine bridge abutments.

Floodplains provide temporary storage space for floodwaters and sediment, giving lag time to a flood event. Flooding is natural and mostly beneficial. Floods are when sediment gets transported, when streambed materials are cleansed and sorted, and when floodplains are nourished with sediments and nutrients. Flood pulses create, renew, and alter the habitat for invertebrate communities, amphibians, reptiles, and fish spawning. And in the fall, waterfowl need floodplain wetlands for foraging habitat .

But, constant bankfull or flood conditions would mean massive scouring, erosion, and high water tables. Low flows give the river channel respite from abrasive flows. Vegetation has a chance to grow on the streambanks, with roots to help hold soil in place and stems and leaves to retard run-off. It is during low flow periods that animals more easily move about so they can maintain populations in several locations. Some aquatic insects develop more rapidly when water velocity is slower and the temperature warmer.

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