FORM AND PROCESS - FLUVIAL GEOMORPHOLOGY AND FLOOD-FLOW INTERACTION, GRANT RIVER, WISCONSIN

Interactions between valley floor morphology and flood discharges are investigated in the 690 km2 Grant River watershed, southwestern Wiscon sin. In this watershed, the conversion from natural vegetation to agri culture resulted in the development of incised meander belts along str eams draining between 10 and 200 km2. These belts consist of alluvial terraces that confine flood flows to a relatively narrow portion of th e valley. The present study examines interaction of this landform and the process of flooding over a range of flood magnitudes. A computer m odel simulates flood flows in two morphological systems: one based on the morphology measured in the field and the other representing a simi lar system without terraces (roughly analogous to pre-European settlem ent morphology). Peak discharges in the case without terraces range fr om 70 to 98 percent of those in the field case. The difference between these two cases varies with the total volume of runoff. Flood dischar ges are rapidly transported through the meander belt in upstream reach es but inundate the wider floodplains of tributary junctions and the d ownstream portion of the drainage system where the meander belt is abs ent. The geomorphological development of the watershed exhibits positi ve feedbacks that maintain the spatial extent of the meander belt; tha t is, overbank sedimentation in headwater reaches with well-developed meander belts is as rare as it is common in the lower reaches of the w atershed.