Abstract
Few studies have examined the fate of dredge spoil placed along river corridors from a geomorphic and retrospective perspective. Such studies can provide guidance to rivers currently being dredged. The Apalachicola River in Florida was dredged from the 1950s for several decades, with disposal on the floodplain, sand bars and open water during the navigation project. One of the larger mounds, disposal site 40 (DS 40) or locally called “Sand Mountain,” originated in an artificial cutoff and stands ~22 m high and extends ~10 ha in area. Because this section of river was a problem for navigation when dredging was being conducted, detailed historical survey sheets with topographic and bathymetric data exist. We analyze the local floodplain and channel geospatially and compile related dredging and disposal data to examine the geomorphic changes of this feature and the adjoining river. Poor placement has resulted in this and other dredge spoil mounds returning sediment to the river through mass wasting and lateral erosion. Intermittent sediment pulses have altered channel morphology adjacent to the spoil mound with the channel becoming wider and shallower than originally and width-depth ratio increasing three-fold. Although dredging of this river has not been conducted for nearly two decades, we found that poor placement of spoil can continue to stress and alter a river with sediment input in the decades that follow. Based on our findings, dredging downstream of distributaries may result in poor outcomes. Locally, dredge spoil placement in cut-off channels close to the river, the outer bend of meanders, and upstream of problem or shallow areas should be avoided. Findings of this study have implications to other anthropogenic deposits including loose material left on floodplains in areas of intensive sand and gravel mining in streams and floodplains.
