However, land area data do not tell the whole story, as subaqueous aggradation must precede land emergence. LP6 has been an area of significant deposition throughout the history of river management on the UMRS (Fig. 6). Between 1895 and 2008, an average of 2.2 m of sediment aggraded in the subset of LP6 for which bathymetric data were analyzed (Table 4). For the 0.34 km2 area, sediment storage increased by ∼750,000 m3. Some areas increased in elevation by up to 6.6 m, while other areas deepened by up to 6.3 m. The greatest aggradation has been in areas find more that have emerged since the 1990s. In particular, the lower portion of lower Mobile Island was the deepest

part of the area in 1895. The river’s right bank and immediately south of the Island 81 complex have scoured most deeply. Degradation of the river

bottom upstream of the present position of upper Mobile Island has also occurred. Between 1895 and 1931, the aggradation rate was 21 mm/yr, resulting in 0.7 m of sediment accumulation. Elevation changes ranged from +3.7 m to −4.0 m during this period, with the greatest accumulations occurring where land emerged attached to Island 81, upstream of upper Mobile Island, and in the area that is now the downstream portion of lower Mobile Island. Areas of degradation mostly corresponded to areas of emergent land in both 1895 and 1931, and are likely the result of uncertainty in assigning land elevations that lacked survey data. The overall estimate of aggradation in this period is likely to be underestimated, since it is unlikely that land elevations were decreasing. Between 1931 this website and 1972, MTMR9 the aggradation rate was 24 mm/yr, resulting in 1.0 m of accumulation. While 5 years of the period occurred before Lock and Dam #6 closure, it is clear that substantial aggradation occurred following closure, and the rate is attributed to post-dam conditions. Aggradation occurred over large swaths of the bathymetric study area, with elevation changes ranging from +3.5 m to −2.4 m. The greatest aggradation occurred at lower Mobile

Island, which emerged above water near the end of the period. Substantial aggradation also occurred at upper Mobile Island, which expanded substantially between 1940 and 1972. Elevation decreases occurred along the right riverbank and upstream of upper Mobile Island. Some decreases may also be attributed to uncertainty in assignment of land elevations in the 1931 dataset, but all occurred where land disappeared and has not reemerged following closure of Lock and Dam #6. Between 1972 and 2008, the aggradation rate was 14 mm/yr, resulting in 0.5 m of sediment accumulation. Thus, sedimentation rate was ∼40% lower in this period than 1931 to 1972 and ∼30% lower than between 1895 and 1931. Similar to earlier periods, elevation changes ranged from +3.2 m to −4.