Preliminary assessment of planform change at low flows with vegetation expansion: Platte River, Nebraska

摘要

A central reach of the Platte River in Nebraska has braided, anastomosed, meandering,and wandering channel form. Proliferation of phragmites (Phragmites australis) andother in-channel vegetation during eight years of low flow may be altering channelshape. Deterioration of braided reaches, as represented by decreases in width-to-depthratios (W/D), could reduce habitat for three endangered and threatened bird species. Inthis preliminary assessment, W/D values for ten cross sections at five study sitessurveyed in 2008, are compared to W/D values from surveys at the same locations in2005, 1998-2001, and 1985. All ten cross sections are on managed lands wherevegetation is controlled and removed. ANOVA and paired t-tests showed nosignificant difference in primary indicator W/Dave between 1985, 2000 and 2005 datasets, but some indication of planform change between 2005 and 2008 (P=0.052). Nochange between 1985, 2000, and 2005, and significant change between years 2005 and2008 (P=0.01) are confirmed using a second indicator W/Dmax. Finding is not the resultof vegetation stabilized banks at the sites, or hydrologic regime, but may be due tosediment-short flows. Recommendations are made to extend this study with surveys onunmanaged lands and at sites of meander planform. Future studies may determine ifvegetation is altering planform through bank stabilization, and assess if sedimenttrapping by vegetation and phragmites in overbank areas is reducing sediment transportduring high flows.