Wild Side: Rushing rivers shape their courses

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The Rush River upstream from the lower bridge in Martell on Sept. 1. Dan Wilcox photo

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Hiking, canoeing or fishing on our local rivers late this summer provides lessons in fluvial geomorphology (the study of physical processes in river systems). The 5-plus inches of rain that came down during the night of Aug. 10 and 11 sent powerful flash floods down the Kinnickinnic and Rush rivers. The effects of those floods are clearly visible.

With the rise in water level in a river during a flood, there is an increase in current velocity and shear stress on the bed. The channel bed tends to scour during high flow. Inflowing rocks, gravel and sediment is deposited as the flood flow recedes, filling the channel.

Channel scour and fill occur over short time scales whereas aggradation and degradation apply to long-term average changes over years. The mid-August flood on the Kinnickinnic and Rush rivers produced some significant channel scour and fill that reshaped the geometry of the river channels. Your favorite trout fishing hole was probably changed by the flood.

In addition to the changes in the river channels caused by the flood, the floodplains were reshaped. Floodplains are relatively smooth strips of land bordering rivers that are overflowed during high water. Floodplains are ecologically rich places with trees, vegetation and wildlife adapted to flooding.

There is a remarkable similarity among rivers in the frequency of bankfull flow. The recurrence interval for bankfull flow on most rivers is between one and two years. When a river is flowing at or near bankfull flow, it has the most power to mobilize sediment in its channel. The mid-August floods on the Rush and Kinnickinnic rivers far exceeded bankfull flow. Both rivers filled their floodplains with raging water, snapped over trees and mobilized tons of sediment and woody debris.

Large floods are ecological re-set events for rivers. A week after the flood, the Rush River discharge was back down to normal and flowing clear. The rocks were scoured of periphyton (algae that grow on the river bed). Spring water flowed in with an azure blue tint imparted by the blue sky and calcium carbonate dissolved from limestone. Trout were still there, having found shelter somehow and returned to their home reach of river. I suspect that many fish were washed well downstream or onto the floodplain during the flood.

By last week, periphyton had re-colonized the rocks and trout were feeding on small emerging mayflies. Many aquatic macroinvertebrates like mayfly larvae burrow downward into the substrate to take shelter during floods. Masses of woody debris, grass, floodplain plants and big round bales of hay are lodged against trees in the floodplain.

The reach of the Rush River in Martell upstream of the lower bridge was modified for trout habitat improvement a couple years ago with a meandering low flow channel, boulders, lunker structures and a cleared floodplain area. The trout habitat features in the low flow channel survived the flood intact. The tall vegetation in the floodplain had been scoured clear and reed canary grass was growing back already.

Aldo Leopold’s son, the late Luna Leopold, was trained as a civil engineer, meteorologist and geologist. His scientific publications spanned 68 years. He greatly advanced understanding of geomorphology, watersheds and river dynamics.

Leopold explained: Just as turbulence in a fluid has a large random component, local variations in erosion and deposition also exhibit random variations having physical effects … that cumulatively forms and maintains the stream channel in all its complexities. The river, then, is the carpenter of its own edifice.

Please send any comments and suggestions for this column to me at wildside@rivertowns.net

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