WOODY VEGETATION AND CHANNEL MORPHOGENESIS IN LOW-GRADIENT, GRAVEL-BED STREAMS IN THE OZARK PLATEAUS, MISSOURI AND ARKANSAS

Woody vegetation affects channel morphogenesis in Ozark streams of Mis souri and Arkansas by increasing local roughness, increasing bank stre ngth, providing sedimentation sites, and creating obstructions to flow . Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions wi th increasing drainage basin area, In upstream reaches, streams have a bundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where va lleys are wider and alluvial banks are higher vegetation has different geomorphic functions, At drainage areas of greater than 100-200 km(2) , Ozarks streams are characterized by longitudinally juxtaposed reache s of high and low lateral channel migration rates, referred to as dist urbance reaches and stable reaches, respectively. Whereas stable reach es can develop stable forested floodplains (if they are not farmed), d isturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes. Disturbance reac hes can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectio nal lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that dens ely vegetated bands of woody vegetation formed depositional sites duri ng bedload-transporting events. The same floods caused up to 20 m of e rosion of adjacent cutbanks, scoured non-vegetated areas between veget ation bands, and increased thalweg depth and definition, In high-energ y (or riffle) zones, channel movement is dominantly by avulsion. In th ese zones, vegetation creates areas of erosional resistance that becom e temporary islands as the channel avulses around or through them, Woo dy vegetation on islands creates steep, root-defended banks that contr ibute to narrow channels with high velocities. Calculation of hydrauli c roughness from density and average diameter of woody vegetation grou ps of different ages indicates that flow resistance provided by vegeta tion decreases systematically with group age, mainly through decreasin g stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with v egetation age.