Sediment in the River

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Photo credits: Peter Williams and Megan Harris

Topic 1: Sediment in the River:

What is sediment? Sediment is the fine silts and clays that make rivers on PEI turn red after a heavy rainstorm. It is the soil washing off bare fields and clay roads into gullies and ditches and eventually into waterways.

Why is sediment in the water a problem? Sediment in small quantities is not a problem in rivers. It can amend the soil for aquatic plants to take root and generally diversify the stream bed structure. The problem is, streams on the Island don’t have just a little bit of sediment. They have tonnes and tonnes of sediment deposited from decades of extreme soil erosion – valuable topsoil lost to the upland habitats. In the stream bed, it buries the gravelly bottoms used by fish to spawn and invertebrates to attach and live. It makes the stream shallower, allowing water tolerant scrub trees like alder and willow to creep in and choke the stream channel. This slows the water down even more and allows more suspended sediment to drop out of the water column. Gradually the stream widens, expanding up onto the banks and creating a flooded plain that has low plant diversity. It also results in loss of land to the surrounding landowners. Eventually the original gurgling, channelized stream is replaced by a wide, slow waterway, densely choked with alder and with low habitat value for aquatic animals.

What do we do to correct this? CQWF tries to flush the sediment out from the bottom of a buried stream bed. We do this by cutting out alder from choked sections, by building brush mats along the edges of the stream to catch sediment during high flow events, and by installing sediment traps or bypass sediment ponds to collect sediment for removal with heavy equipment. In one season, we have been able to clear sediment that was over a metre deep from the stream bed, until the gravel bottom was once again visible and available for fish and invertebrates to use.

What is a brush mat? It is a simple structure built usually from conifer tree boughs (some watershed groups use your discarded Christmas trees for this), wooden stakes and binder twine. The tree boughs are placed along the edge of the stream, staked and tied into place so they don’t move, and angled up until they meet the top of the stream bank. These boughs catch sediment as it flows by when the water level is high (spring and fall) and thereby permanently take it out of the main stream channel. A watershed group has to have the technical expertise to know exactly where to place these brush mats along a meandering stream channel, to be efficient at collecting sediment.

What are sediment traps and bypass sediment ponds? These are both just deepened sections of the stream. A trap is right in the stream, whereas a bypass pond is adjacent to the stream. The terms sound pretty technical, but in reality these are just big holes dug with heavy equipment. The stream flows into these deeper sections and, because they’re deeper, the water slows down. When it slows down, any sediment the water was carrying drops out to the bottom. Gradually, the amount of sediment collected in these holes fills the hole and it has to be scooped out with the heavy equipment again. CQWF has one sediment trap and one bypass pond in Brookvale. Both of these structures filled with sediment in just one summer – spring season! However, the sediment trapped here represented a century of erosion sitting in the bottom of the stream for decades. After the first clean-out and provided that today’s erosion from fields and clay roads is not too severe, sediment traps should only need cleaning every few years.

View Topic 2: Adding Cover Habitat for Fish: Click here.

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