EFFECTS OF LAND-COVER HISTORY AND GEOLOGIC SETTING ON CHANNEL PROCESSES, LAKE SUPERIOR SOUTH SHORE STREAMS

The U.S. Geological Survey conducted channel geomorphic assessments for several Lake Superior tributaries in Wisconsin and Minnesota over the last decade to determine natural and human causes for excessive sedimentation and aquatic habitat degradation and to provide base-line data for stream protection and rehabilitation. Studied streams included unregulated ephemeral and perennial reaches draining forested, agricultural, and urban watersheds of less than 1 square kilometer to greater than 1,000 square kilometers. Increased runoff from widespread clear-cut logging and burning in the late 1800s and subsequent agriculture in the early 1900s caused gullying, landslides, bank erosion, channel avulsion, overbank sedimentation, and aggradation. Local channel responses to increased runoff depended on the composition of surficial and subsurface glacial deposits and bedrock, type of glacial or glacio-lacustrine landforms, spatial position within the watershed, and valley development. Responses were most evident following large floods. Runoff from upland surfaces presently is still elevated above pre-settlement rates because of differences in forest cover type and age, changes in drainage network patterns from road drainage and agricultural ditching, and urbanization.

The major source of sediment in most Lake Superior south-shore streams has been and continues to be sand from landslides and mass wasting of valley sides where steep channels intersect relict glacial lake shorelines from approximately 220 to 320 meters above sea level. Overbank sedimentation and natural levee formation affect downstream channel morphology, floodplain connectivity, and riparian vegetation. Sandy historical overbank deposits are 1-3 meters thick, making entrenched channels look and hydraulically behave like incised channels. Storage of historical overbank sediment and its effects on channel processes can last for centuries and millennia. Reducing runoff and sedimentation and restoring flood-plain connectivity are challenges facing rehabilitation efforts.