Flooded out land from a pond.

Controlling Stream EROSION and Sediment LOADING in Ponds

May 2019

Spring flooding in the northeast and elsewhere, as snow melt and heavy rains occur, triggers major stream erosion and sediment loading in ponds. Large volume runoff and downright flooding isn't unusual in spring. But add climate change, and higher than average flooding causes higher than average pond damage. 

The result of such erosion and loading can be damage to inflow and outflow channels; shallowing of the pond; changes in actual pond shape as sediment alters the shoreline; growth of aquatic plants and algae in new shallow areas; and water warming due to shallowing and reduced water volume.

How do pond owners prevent sedimentation of the pond basin and erosion of inflows and outflows? And the expense of repairs and clean-outs?

Most ponds require some kind of in-flowing natural water source, in addition to in-pond springs/ground water, well water, roof catch, etc. In other words, water flowing by gravity off the pond watershed. This may be runoff from rain and snow melt, or an actual stream flowing into the pond, or both. A small stream can be a good water resource. But the bigger it gets, the more it may cause problems. That's why a good general rule in pond building and siting is not to run a large stream directly into the pond. Not only can it cause sediment loading, flooding, and erosion problems, but it may be a violation of state and/or federal wetland laws.

So for new pond builders, I caution against running a stream directly into the pond unless it's small and unlikely to cause problems. If possible, divert the stream around the pond and for supply construct a controllable feed piped or channeled from the stream. Or locate the new pond beside the stream. 

But in some situations, a direct inflow stream may be desirable or necessary for water supply.

How can you set this up to cause the least problems? The stream should be modified to minimize erosion. This may be done by reshaping the channel to reduce erosion potential, for instance flattening the sides, and adding edge plantings to help hold the soil together. Creating small steps and waterfalls can help reduce stream velocity and erosion. Curves rather than straight runs can slow down the stream flow. Adding rip rap or stone is often done to reduce erosion, sometimes over a geotextile lining. 

A catch pool in the stream just above pond can trap sediment before it flows into the pond. Catch pools are built in variety of ways depending on stream size, from a simple

dugout basin to more elaborate boxes, cisterns, etc. They will need to be cleaned out periodically, often by backhoe or excavator, and should be designed with that in mind, as well as access, and disposal of sediment.

Keep in mind that all the above mentioned stream work alterations may require state or federal permitting. Know the law.

Another way to catch sediment delivered by stream is with an in-pond silt pool. This requires digging a crater-like catchment where the stream enters the pond. It is below water level, a pool within the pond, and often incorporates a barrier or "speed bump" at the far end opposite the stream entry to help trap sediment. This submerged catchment requires no stream alteration, and must be cleaned out periodically, like other types of silt pools. Because the trap is submerged, the pond may need to have its water level lowered to expose the trap for efficient cleaning and maintenance of the structure.

If your pond is already constructed and includes a direct inflow stream, consider the preceding 

strategies to reduce sediment loading. 

Your pond discharge system is also vulnerable to high water erosion and other damage. If discharge is through an offshore pipe such as a standpipe/drop inlet it should be free of sediment problems. But be on the lookout for floating debris collecting around the inlet or causing plugging, and remove debris. If the discharge pipe is a water level horizontal or slanted culvert at the shoreline it will also be vulnerable to debris plugging, as well as sediment loading. Keep it clean.

Read More Pondologies

Topics vary from month to month and provide great information when researching where you want to build a pond, or how to keep it clean.  Other areas of interest are pond use for more than swimming, and how to keep fish happy in your pond.