Nonlinear Downstream Changes in Stream Power on Wisconsin's Blue River

Abstract

Spatial characteristics of stream power are examined along four stream reaches through the 208-km2 Blue River watershed in southwestern Wisconsin. Although prevailing theoretical arguments often use spatially aggregated data and power functions derived from hydraulic geometry to suggest that stream power increases linearly downstream, analysis of stream-power trends along the Blue River's spatially connected stream pathways in southern Wisconsin shows that these pathways exhibit varying degrees of nonlinearity in the downstream distribution of stream power. In three of the four stream pathways, cross-sectional stream power and mean stream power reach maximum values at drainage areas between 10 and 100 km2 before decreasing rapidly downstream. Geomorphic and lithologic controls, which produce unusually steep channel slopes in mid-basin locations, are primarily responsible for the nonlinear downstream trends in stream power. Wide valley bottoms and gentle channel gradients occur in erodible lithologies, while resistant lithologies are associated with narrow valley bottoms and steep channel gradients. Spatial variations in drainage network evolution and past stream flow and sediment conditions also influence modern channel gradients. Downstream variations in cross-sectional stream power along interconnected pathways within the drainage network influence the quantity of post-settlement overbank sediments stored on floodplains. Greater sediment storage occurs in the headwaters and lower valleys where stream power is low, whereas little sediment is stored in mid-basin reaches with high stream power.