HISTORICAL DISTURBANCE AND GEOMORPHIC RECOVERY OF BIG RIVER, EASTERN MISSOURI

River systems can take decades to centuries to recover from the effects of anthropogenic disturbances. However, few studies document the dynamics of the geomorphic recovery process. This study evaluates channel recovery trends along 180 km of Big River in the Ozark Highlands in eastern Missouri using historical aerial photographs from 1937, 1954, and 2007 to quantify changes in gravel bar area and lateral channel movements. Historical land use disturbance peaked between 1880 and 1920 in the region. During this period, up to 2 m of overbank sediment was deposited on floodplains and large gravel bars began to form in response to increased upland soil erosion and tributary sediment delivery. Additionally, excessive amounts of sand/gravel-sized Pb-Zn mine tailings were discharged into upper segments of the study area between 1900 and 1945. Results show that the recovery process is complex in the Big River. Bar frequency and bank erosion rates have not decreased since the 1930s. In fact, large floods and/or tributary incision may have increased geomorphic activity to record levels in recent times. Active and stalled bed waves occur in most segments and they are best expressed in the mining area, possibly due to the greater bed mobility of the finer tailings materials. Lateral channel erosion is occurring in association with both new floodplain formation (recovery process) and channel widening (disturbance response). Presently, bank erosion is a net source of fine-grained sediment to the Big River and the primary source of remobilized mining-contaminated sediment to the Big River.