Preliminary Assessment of Planform Change at Low Flows with Vegetation Expansion: Platte River, Nebraska

A central reach of the Platte River in Nebraska has braided, anastomosed, meandering, and wandering channel form. Proliferation of phragmites (Phragmites australis) and other in-channel vegetation during eight years of low flow may be altering channel shape. Deterioration of braided reaches, as represented by decreases in width-to-depth ratios (W/D), could reduce habitat for three endangered and threatened bird species. In this preliminary assessment, W/D values for ten cross sections at five study sites surveyed in 2008, are compared to W/D values from surveys at the same locations in 2005, 1998–2001, and 1985. All ten cross sections are on managed lands where vegetation is controlled and removed. ANOVA and paired t-tests showed no significant difference in primary indicator W/D_ave between 1985, 2000 and 2005 data sets, but some indication of planform change between 2005 and 2008 (P=0.052). No change between 1985, 2000, and 2005, and significant change between years 2005 and 2008 (P=0.01) are confirmed using a second indicator W/D_max. Finding is not the result of vegetation stabilized banks at the sites, or hydrologic regime, but may be due to sediment-short flows. Recommendations are made to extend this study with surveys on unmanaged lands and at sites of meander planform. Future studies may determine if vegetation is altering planform through bank stabilization, and assess if sediment trapping by vegetation and phragmites in overbank areas is reducing sediment transport during high flows.