Sources, Distribution and Effects of Sediments

When a pond is first constructed, the owners are rarely thinking ahead to a time when dredging may be necessary to allow its continued survival. Sediment accumulation is, however, the principal reason that ponds "age" and disappear in this region (sometimes ponds are deliberately removed as a part of changing land use, or to restore stream habitat). Just how rapidly this aging takes place isn't completely understood, but depends on pond size, the presence or absence of stream inputs, and wise management of the watershed. This section considers the effects of sediment-induced aging

Ponds are typically distinguished from lakes primarily on the basis of their small area and shallow basins. Small size, unfortunately, means that there is less space to store sediments, and a greater portion of a pond becomes filled annually with sediment than is true of larger lakes. As a result, sediment control is an important, long-term management necessity.

Sediments arrive primarily via streams and overland runoff. The particles are mostly eroded constituents of soil, and their rate of movement toward ponds is accentuated by activities within the watershed that involve soil disturbance. Steep slopes, adjacent to the pond, result in greater sediment inputs. Land cover is also important. Woodlots typically allow less soil erosion, whereas tilled agricultural land often exports large amounts of soil to ponds.

The soil particles come in all sizes (e.g., sand, silt and clay), and may be organic (e.g., leaves) or inorganic. Toxic materials (e.g., metals) may in rare instances enter ponds as well. Upon arrival, most sediments settle to the bottom (ponds with unusually short hydraulic retention times [HRT = pond volume/ discharge at the outfall] rapidly convey water downstream, and may be less efficient at trapping sediments). Bigger particles (e.g., sand grains) settle out first, and it is typical to find sandier sediments near any inflows and in shallow water near shore. By contrast, very fine particles remain suspended in the water column for a longer period of time (and are easily resuspended during storms). The sediments in deeper water near the middle of the pond or nearer the outfall (standpipe or dam) are usually fine grained as a result. The presence of aquatic plants helps to reduce water movement, and thus accelerates the rate of sediment settling.

As sediments accumulate, the pond becomes progressively shallower, and the bottom is increasingly exposed to greater light intensity. This often allows aquatic plants to grow much more rapidly, and pond owners often take notice when submersed plants such as pondweeds start to become obvious at the surface. As stated in Section IV, moderate abundances of aquatic vegetation are a very healthy contribution to pond ecosystems. If plants become dominant throughout the pond, however, their presence can produce large fluctuations in oxygen (consumed at night in large part by the very plants that produced it during daylight hours). The kinds of plants, and the animals they support, can be expected to change over time as well. In effect, sediment accumulation is the principal cause of "ecological succession" in most ponds.

Sediments may also be important as a source of nutrients, especially phosphorus. Rooted plants rely on the P-rich sediments for most of their nutrition. Storms may also resuspend smaller sediment particles, which release some of their phosphorus directly to phytoplankton in the water column. Don‘t be surprised to see more plants and algae during wet summers.