EFFECTS OF VEGETATION GROWING ON UPPER POINT-BAR SURFACES ON BAR MIGRATION, OUTER BANK EROSION, AND CHANNEL EVOLUTION: OBSERVATIONS OF THE GRAVEL-BED MAD RIVER, WEST-CENTRAL OHIO, USA

Although many studies have examined the effects of riparian vegetation on rivers and floodplains, few, primarily laboratory studies, have addressed the effects that vegetation growing on bar surfaces has on bar migration and channel evolution. The gravel-bed Mad River in west-central Ohio is a field example of a river whose evolution has been influenced, in part, by vegetation on bars. The Mad River flows through a low-relief valley that is filled with glaciofluvial outwash and a thin veneer of Holocene fluvial deposits. In the study reach, bankfull width is on the order of 50 m to 70 m, and bankfull depth is on the order of 3 m to 4 m. The study reach had been channelized and straightened more than 50 years ago. However, because there are no dams on the river, it experiences bankfull flows and the study reach has evolved to a natural state, exhibiting meanders, point bars, and braid bars. Vegetation, primarily tall grasses and weeds with scattered small saplings, colonizes upper point-bar surfaces during years that do not experience bankfull flows. If the vegetation becomes well established, persisting and propogating from year to year, subsequent bankfull flows may fail to remobilize sediment within the vegetated areas. The vegetation apparently reduces flow velocities and redirects flow toward the outer bank. These effects essentially mimmick a forced reduction in bankfull channel width. Because channel width, depth, and sinuosity scale with flow under bankfull conditions, and point bars scale with channel width and depth, the effective reduction in channel width promotes outer bank erosion, point bar migration, and an increase in sinuosity (i.e., vegetation promotes meandering). These results suggest that controlled plantings of vegetation on bar surfaces in other rivers may be useful in river restoration. Conversely, for rivers where outer bank erosion is not desirable and dams are located upstream of the reach of concern, occasional (e.g., once every one to two years) bankfull flows should be released to prevent or inhibit vegetation from becoming well established on bar surfaces. Additionally on the Mad River, the height of vegetation varies greatly with the seasons. Hence, on the Mad and similar rivers, the effects of vegetation on the flow will depend on the time of year that bankfull flow occurs.