The Great Red River Raft and its Sedimentological Implications - Reconstructing Human-Landscape Interactions: Interpreting Desert and Fluvial Environments - page 50

partially severed. These bendways were later developed in the early AD 1900s for
recreational and environmental purposes. According to Robinson ( 1995 ), exces-
sive silting is still problematic downstream of some of the bendways.
Sediment Management
The placement of a dam generally reduces the overall flow velocities of a river and
in turn, tends to induce sediment deposition. Siltation can occur in the approach
channels or the actual lock chamber; thus, dredging is needed to maintain the
required 9 foot navigation channel. The fine-grained sediment in the transported
load is capable and often largely responsible for impeding navigation. The higher
concentration of fine-grained sediments in a transported load results in an
increased need for maintenance dredging. The Red River, in particular, carries a
large amount of fine-grained suspended sediment (Combs et al. 1994 ).
Red River bank materials originate from sources that include meander belt
alluvium and clay plug materials, back-swamp deposits from a nineteenth-century
flood plain, and Pleistocene/Tertiary materials from the alluvial valley walls.
Generally, scour depth increases with outer bank resistance to erosion and failure.
Scour pool depths for revetted bends with non-erodible outer banks are 5-20%
greater than those in equivalent free, alluvial meanders (Thorne 1992 ).
Sound understanding of the processes and mechanisms involved in bank ero-
sion is very important since it is believed to be the main source of the fine bed load
sediments. Bank stabilization efforts in the reaches above Shreveport have greatly
reduced the sediment problem. However, some river responses to the bank sta-
bilization efforts, which include increased bed scour in revetted bends, reduced
sediment storage capacity in crossing bars, and enhanced sediment transport owing
to channel realignment, can produce an increase in sediment load from other
sources. These concerns have to be considered in order to assure the long term
decrease in sediment load. Protected bank materials could be replaced by the bed
load sediments as bed scour in revetted bends increase in an attempt to cancel the
effect of reducing long term sediment load in the river. When the outer banks are
revetted, the point of maximum weathering migrates downstream at high river
flow and overlaps the revetment of the next bend. The channel capacity to store
sediments in the mid-channel bar between bends is reduced, which results in lower
water elevations during floods and increased navigation depth during low flows at
the expense of faster sediment transfer downstream.
Furthermore, river sinuosity can have significant effects on the sediment load of
the river. When channels are realigned, the sinuosity decreases while the channel
gradient increases; this can result in a substantial increase in sediment load. In the
case of the Red River, this increased sediment load would be supplied by bed
scour. The reduction of flow resistance by the straightening of the channel would
increase the flow velocity and thus, the sediment transport capacity, thereby having
an even more dramatic effect on increasing the sediment load than the one