FLOODS AS NATURAL AGENTS OF RIVER COMPLEXITY: PHYSICAL CONTROLS AND GEOMORPHIC IMPLICATIONS (Invited Presentation)

Sediment and debris transported by floods create complexity through bank erosion, landslides, and floodplain deposits in both managed and unmanaged rivers. We propose that systematic and potentially predictable changes occur during flood events that both create and maintain complexity. Specifically, downstream gradients in sediment transport affect accumulation and removal of sediment and organic matter along channel margins in accordance with conservation of mass and the Exner equation. Thus, downstream undulations in stream power may create alternating environments along river courses. Using Mink Brook in New Hampshire as a natural laboratory with moderate flood events, we determined stream power gradients though GIS analysis of 10-m digital elevation models (DEMs) and selected 6 cross sections over 9 river km alternating between increasing and decreasing stream power. We found that near-channel locations at the ~2- to ~25- year flood elevation had thinner alluvial cover (average 23 cm), lower ²¹⁰Pb_ex inventories (70 to 1,000 bq/m²), lower organic matter inventories (17 to 219 kg/m²), and steeper topography at cross sections in reaches of increasing stream power when compared to those of decreasing stream power which had thicker alluvial cover (average 41 cm), higher ²¹⁰Pb_ex inventories (up to 9,100 bq/m²), higher organic matter inventories (up to 3,300 kg/m²), and flatter topography. These data suggest that alternating reaches of increasing and decreasing stream power create geomorphic complexity on the kilometer scale, with alternating erosional or depositional modes. The different styles of disturbance yield characteristically different near-channel topography. Results of moderate flood events on Mink Brook are consistent with recent studies on extreme floods in Vermont and Colorado which show erosional and depositional features associated with kilometer scale reaches of increasing and decreasing stream power, respectively. This work has broader implications for riparian ecology and river restoration by demonstrating physical forcings in flood events that create complexity.