Form and Process: Fluvial Geomorphology and Flood-Flow Interaction, Grant River, Wisconsin

Abstract

Interactions between valley floor morphology and flood discharges are investigated in the 690 km² Grant River watershed, southwestern Wisconsin. In this watershed, the conversion from natural vegetation to agriculture resulted in the development of incised meander belts along streams draining between 10 and 200 km². These belts consist of alluvial terraces that confine flood flows to a relatively narrow portion of the valley. The present study examines interaction of this landform and the process of flooding over a range of flood magnitudes.

A computer model simulates flood flows in two morphological systems: one based on the morphology measured in the field and the other representing a similar system without terraces (roughly analogous to pre-European settlement morphology). Peak discharges in the case with out terraces range from 70 to 98 percent of those in the field case. The difference between these two cases varies with the total volume of runoff.

Flood discharges are rapidly transported through the meander belt in upstream reaches but inundate the wider floodplains of tributary junctions and the downstream portion of the drainage system where the meander belt is absent. The geomorphological development of the watershed exhibits positive feedbacks that maintain the spatial extent of the meander belt; that is, overbank sedimentation in headwater reaches with well-developed meander belts is as rare as it is common in the lower reaches of the watershed.

Key Words:

• floodplains
• floods
• fluvial geomorphology
• sedimentation
• terraces
• Wisconsin