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<br />small flood events. For a single structure, the greatest reductions will
<br />occur immediately downstream from the pond and the effect of the pond
<br />will continually decreas e in a downstream direction.
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<br />The,downstream effects of stock ponds on sedimentation can be
<br />highly variable depending uponthreeinfluences. These are: (1) their
<br />location relative to major sou.rces of sediment in a drainage basin; (2)
<br />the trap efficiency of the stock ponds in the area; and (3) theposs ibili-
<br />ties for re-entrainment of sediment by channel erosion below the stock
<br />ponds.
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<br />In many sections of the Southwest, the major sediment source
<br />consists of erosion in alluvial channels. Stock ponds, on the other hand,
<br />are frequently located on small upland tributaries where this type of ero-
<br />sion has not developed and sheet erosion is minor in nature. In such
<br />instances, the presence of stock ponds has little effect on sediment con-
<br />centrations downstream. Where sheet erosion is a significant contributor
<br />of sediment, the stock ponds may cause some reduction in sediment yield,
<br />depending on the percentage of total watershed area above them.
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<br />The ability of stock ponds to trap a large portion of the sediment
<br />delivered to them, termed their trap efficiency, can have a significant
<br />effect on downstream concentrations. Occas ionally ponds are built large
<br />enough to hold all or most of the water during infrequent flood events. In
<br />such instances, practically all sediment will be depos ited and only small
<br />quantities of clear water will be released. Where capacities provide
<br />storage only for annually recu rring events, a large volume of sediment
<br />may pass over the spillway during the less frequent events, since the
<br />higher concentrations of silt occur at 'such times. As the capacity of
<br />ponds is progress ively reduced with time, their efficiency in trapping
<br />sediment also reduces. The time factor causes stock ponds to ,have a
<br />continually decreasing downstream influence, in areas of extensive ero-
<br />s ion, but it has only a minor influence in areas of limited eros ion.
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<br />Deposition of sediment in stock ponds may cause increased ero-
<br />sion of the channel downstream because of clarifying the water as it is
<br />stored in or passes through the pond. This phenomenon can be of impor-
<br />tance if erodible material is available in the downstream release channel.
<br />The sediment transport capacity of any stream is complex in nature.
<br />Whether or not' a downstream recovery of se,diment deposited in a stock
<br />pond will occur depends upon many factors, such as texture and erodibility
<br />of the material through which the discharge flows.
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