Sediment Accumulation Rates


Sediment Accumulation Rates
        Ponds serve as “collection points” in the landscape for sediments derived from soil erosion. Although the in-filling of lakes and reservoirs has received considerable attention, few studies have been directed at sediment burial rates in small ponds, despite the fact that ponds greatly outnumber larger water-bodies in most regions and are frequently built explicitly for sediment retention (but see Renwick et al. 2005; Downing et al. 2008; Downing 2010).
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Fig. 5. Metal probe used to determine current, and original, water depth at regularly spaced locations within each pond.


        The 10 ponds evaluated during summer 2009 all had known construction dates; 5 ponds had stream inflows, and 5 had no stream inputs. A metal probe (Fig. 5) was used to determine the current water depth, then inverted and pushed to the original pond bottom to measure the original water depth) at 32 to 44 GPS-indexed locations. The measurements for each pond were then integrated to compute the amount of sediment that had accumulated since the time of original pond construction. Bathymetric maps were also created to show the depth contours of the original and current pond basin (Fig. 6).
        Annual rates of sediment burial, expressed per unit of pond area, declined with increasing pond size (Fig. 7). This means that owners of small ponds can expect their ponds to fill in more rapidly and require dredging or other management earlier or more often than owners of larger ponds. As shown in the figure, ponds receiving stream inputs also accumulated sediments more rapidly than ponds without inflows. Stated differently, ponds with inflows derive much of their sediment from material conveyed by the streams, particularly during storms.


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Fig. 6. Original (L) and current (R) depth contours of one of the 10 ponds sampled for sediment characteristics and accumulation rates.




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Fig. 7. Per unit pond area-specific annual sediment accumulation rates (based on sediment volume), related to pond area in ponds with stream inflows (blue circles and line) vs. without inflows (green squares and line).


        The watersheds from which the ponds largely derived their sediments (a portion of the material is produced by algae and plants within the pond) may be determined by examining the “high ground“ surrounding the pond. Size, land use and slope are particularly important watershed features. For example, the pond shown in Figure 8 has a relatively small watershed relative to its own size. Much of that watershed consists of meadow (and therefore experiences less erosion than would have been the case with cornfields, for example). The influent stream transports sediments relatively quickly (compare to transport by overland runoff) to the downstream pond shown, but receives a measure of protection in being surrounded on both sides by riparian trees.

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Fig. 8. Watershed features of a pond in the Brandywine drainage, Chester County.


        In general, ponds with larger watersheds experience greater quantities of sediment in-filling (as well as nutrient runoff). However, rates of sediment burial, when expressed per unit watershed area as in Figure 9, typically decline as watershed area increases. This occurs because a portion of the sediment that is eroded from soils more distant from the pond are intercepted by swales or other low-lying areas. indicate that as watershed area increases, each hectare of watershed area contributes less to the pond.

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Fig. 9. Per unit watershed area-specific annual sediment accumulation rates (based on sediment volume), related to pond area in ponds with stream inflows


        In effect, ponds can have very different “life-spans” according to their size, the land area that drains to them, and the presence or absence of a stream connection; sediment removal to extend the life of a pond is more frequent in smaller ponds with stream inflows, and is an important consideration for landowners, managers and conservation organizations. As ponds age, they become shallower and typically support more rooted plants, which in term help to trap sediments by slowing down water movement. Although bank erosion during pond construction may initially contribute substantially to sediment burial, stabilization of the shoreline by natural vegetation can help considerably to reduce subsequent in–filling. An example of the oldest pond among those studied, with a well–developed and diverse assemblage of aquatic and riparian plants, is shown in Figure 10.

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Fig. 10. A well established pond in the northern portion of the Brandywine watershed, with a diversity of plants that (with the exception of the pink water lilies) naturally colonized the shoreline and deeper water. The additional protection provided by the riparian vegetation has done much to increase the pond’s longevity.