Each line in Fig. 9 represents the minimum bed elevation through time for an individual cross-section within the reach. The upstream channel has adjusted to the new hydrologic regime of the dam over a few decades. Fig. 9A shows the bed essentially

stabilized by about 1975. The upper section of the river shows no change from the 1975 flood (1956 m3/s in Bismarck, ND). The lower section has not achieved a new equilibrium following dam completion. The maximum depth of the thalweg did not stabilize until the mid-1990s in the River-Dominated Interaction reach and remains more active than the Dam-Proximal reach (Fig. 9B). Of the 66 major rivers analyzed, 404 dams were located on the main stem of 56 of the rivers. Fifty of these rivers had more than one dam on the river creating a total of 373 possible Inter-Dam sequences. The average distance between these dams is 99 km MK-2206 research buy (median less than 50 km and the range is 1 to more than 1600 km). Thirty-two percent of the Inter-Dam sequences had lengths of 25 km or less, 41% were Selleck ABT-263 less than 100 km, and 26% of the dams were within 1000 km of one another. Only one Inter-Dam Sequence was identified to be longer than the 1000 km. These results suggest that there are numerous large dams occurring in sequence on rivers in the US. Results of this study suggest that the two

dams in the Garrison Dam Segment interact to shape the river morphology, although it is important to distinguish the interaction does not control the entire segment, and some sections only respond to one dam. Five geomorphic gradational zones were identified in the segment between the Garrison Dam and the Oahe Dam and three are influenced by this interaction. The major impacts on channel processes downstream of the Garrison Dam are identified: (1) erosion from the bed and banks immediately below the dam as a result of relatively sediment-free water releases, (2) localized deposition farther downstream

however as a result of material resupplied to lower reaches from mass wasting of the banks, tributary input, and bed degradation, and (3) the capacity for large floods and episodic transport of material has been limited. Similarly, the predicted upstream responses of the Garrison Segment to the Oahe Dam are: (1) the creation of a delta in a fining upwards sequence that migrates longitudinally both upstream and downstream. (2) The sorting by sediment size as velocities decrease in the reservoir. Previous studies on dam effects suggest that these effects will propagate and dissipate (downstream or upstream respectively) until a new equilibrium is achieved. In the Garrison Dam Segment, the downstream impacts reach the upstream impacts before the full suite of these anticipated responses occur. As a result, there are a unique set of morphologic units in this reach. The Dam-Proximal and Dam-Attenuating reaches are not affected by any dam interaction.