EFFECT OF SAND SPLAY COMPLEX FORMATION ON THE TOPOGRAPHIC COMPLEXITY OF FLOODPLAIN HABITAT, COSUMNES RIVER, CA

Sand splay complex formation creates topographic complexity and enhances habitat by locally affecting floodplain inundation duration and frequency. Additionally, increased form roughness associated with splay lobe and lateral levee deposition and new floodplain channel incision creates heterogeneous flow velocity and shear stress. Two intentional levee breaches intended for habitat restoration at the Cosumnes River Preserve, CA, provide an opportunity to monitor geomorphic processes and sand splay complex formation that create the physical structure of new floodplain habitat. Post-breach field data include detailed EDM surveys of sand splay and channel complex topography. Quantitative measures of variation in floodplain topography allow comparison of floodplains leveled for agriculture - the starting point for the Cosumnes River Preserve study areas - and topography initiated through restoration of flow through breaches. One measure of topographic complexity created by sand splay complexes, C_fp, the ratio of the distance along the new floodplain surface (l_fps) to the distance along a horizontal line (l_h), varies as deposition and incision modifies topography. Quantified as a dimensionless parameter: C_fp=(l_fps/l_h - 1)100, C_fp reached 2.3, three years after the breach at the Corps Breach study site, and reached 3.0, five years after the breach at the Accidental Forest study site. Large woody debris transported through the breach on to the floodplain influences topographic variation in splay channel profiles and enhances topographic complexity of floodplain habitat created by sand splay complex deposition. Maximum relief associated with large woody debris reached 0.7 m, over six times greater than the average relief between low points and adjacent downstream high points (0.11 m). The inherent mobility of large woody debris adds to the dynamic physical diversity of habitat structure created by prograding splay channels and splay complex accretion and incision. Understanding and quantifying the dynamic nature of topographic complexity has application in assessing floodplain restoration projects in lowland floodplain rivers.