These depositional locations are prone to islands because they are well connected to the main channel, even during low flows. This connection can provide a constant supply of sediment from the main channel, yet flow velocity is reduced, dropping the sediment out of suspension
(Ashworth et al., 2000 and Tal and Paola, 2007). Such conditions for CP-673451 datasheet island growth well-describe LP6. Less than 2.5 km upstream of LP6, the river is <950 m wide, so it widens ∼60% into LP6. Further the secondary channel in which the Mobile, Gull, and other islands have emerged is about 50% wider than the navigation channel to the north of Island 81. Thus, LP6 has appropriate geometry for island building, without excessive Selleckchem AZD2281 wind-fetch width to spur wave erosion. Wing and closing
dikes result in depositional and erosional environments that were not present prior to river management (Pinter et al., 2010 and Alexander et al., 2012). By reducing velocity in secondary channels, closing dikes promote deposition. The principal area of island growth in LP6 occurs downstream of both a closing dike and a wing dike and atop and upstream of a second wing dike. This pair of wing dikes may have served as barriers promoting deposition by further slowing water already affected by the closing dike. Evidence that the structures have influenced deposition patterns is found in the growth of Gull and Mobile Islands in areas where land did not exist in 1895. Recent growth of land in sectors 4, 5, 9, and 10 (Table 3, Fig. 5) suggests that close proximity to wing and closing dikes is not necessary. These growth areas are immediately
upstream of Lock and Dam 6 and may be a function of sediment trapping by the dam. If similar wing and closing dike structures are present in secondary channels in lower pools elsewhere in the UMRS, they might become places where land has emerged or will emerge in the future. Unfortunately, not all structures present in secondary channels are indicated on current navigation charts (http://www.mvr.usace.army.mil/Missions/Navigation/NavigationCharts/UpperMississippiRiver.aspx), ROS1 making it difficult to identify where island-promoting structures may occur. However, in Pool 7, there is a side channel with 19 wing dikes between 0.8 and 4 km upstream of the Lock and Dam. In this channel, aerial imagery indicates growth of Dresbach Island and emergence of new islands in the last 20 years. A short distance upstream, one closing dike and 6 wing dikes on a side channel between Dakota Island and the Minnesota shore have not resulted in growth or emergence in the last 20 years. These examples indicate that structures alone may not be sufficient to facilitate emergence in the absence of other promoting factors. USACE restoration efforts include creating rockfill island “seeds.