EFFECTS OF EPISODIC OVERBANK SEDIMENTATION ON CHANNEL MORPHOLOGY, FLOODPLAIN CONNECTIVITY, AND RIPARIAN VEGETATION

An important aspect of understanding the sediment budget of a stream system is quantification of episodic overbank sedimentation and long-term storage of sediment in floodplains near channel margins. Accelerated overbank deposition and natural levee formation can be substantial along streams that experience frequent overbank flows, are characterized by high sand fraction sediment loads, transition downstream to low gradient alluvial valleys, have forested riparian areas, and have experienced modern and/or historical anthropogenic disturbances. Episodic overbank deposition and long-term storage of overbank sediment affect channel morphology, floodplain connectivity, and riparian vegetation.

Overbank sedimentation rates and volumes were quantified for forested watersheds along the southern shore of Lake Superior, Wisconsin as part of a broader investigation of sediment sources and sinks and geomorphic assessments related to fish habitat and stream rehabilitation. Valley cross-section surveys and floodplain/channel sediment coring were done at multiple locations to characterize the down-valley distribution and timing of overbank sedimentation. A variety of sediment age-dating techniques were used to establish sedimentation rates, including radiometric analysis, soil development, vertical changes in sediment textures that recorded floods of a known age, streamflow records, historical maps, and aerial photographs.

Whittlesey Creek is a small tributary to Lake Superior with accelerated historical overbank sedimentation along its lower main valley. Over 230,000 metric tons of sediment is stored in overbank areas and floodplain of Whittlesey Creek where the channel slope is less 1 percent. Natural levees are up to 1 meter higher than the surrounding floodplain. Excess sediment built up along the banks gives the Whittlesey Creek channel an entrenched appearance, and this has resulted in difficulty fitting the channel morphology into commonly-used river classification schemes. The riparian zone is dominated by disturbance tolerant shrub and herbaceous species because of the high sedimentation rates. Results from the study are being used to develop rehabilitation strategies and plans for improving and preserving southern Lake Superior riparian and estuary habitats.