Stormwater and Sediment

Sediment comes from many sources in our developed environments, including bare soil on construction sites, poorly maintained dirt and gravel roads, and anywhere soil is disturbed. Stormwater runoff acts as the primary transporter of sediment, carrying it from unprotected soil surfaces to the nearest waterway. The hard surfaces we humans create, coupled with exposed soil vulnerable to erosion or runoff, increases the sediment load that enters our waterways. 

The negative effects of excess sediment means reduced sunlight for aquatic plants, less oxygen for aquatic animals, and a rise in water temperature, with ripple effects for all living things in the stream. Sediment buildup on the stream bed degrades this essential habitat for aquatic insects, fish, amphibians, and other wildlife. Pollutants such as heavy metals, excess nutrients, and bacteria attach to sediment particles and are carried downstream. 

The excess sediment in our streams results in increased turbidity – a measurement of how clear the water is. Clear, colorless water has a low turbidity; murky or muddy water has high turbidity, an indicator of the amount of sediment particles suspended in the water. Turbidity is a great indicator of water quality – not just for the presence of soil particles, but also for bacteria and other contaminants. From May to September, clean water agencies use turbidity to measure bacteria levels in the French Broad River at Pearson Bridge. Learn more here. 

As you stroll alongside a stream on your property or a nearby park, have you noticed chunks of soil and vegetation disappearing from the stream’s edge? Streambank erosion refers to the gradual removal of soil, rock, and vegetation from a stream bank, creating a channel or gully. Erosion is a natural geological process, but again – stormwater runoff from impervious surfaces (parking lots, rooftops, and roads) amplifies the volume and velocity of water entering the nearest stream, resulting in excessive erosion. In urban and other developed areas, this fast-moving water can speed up the erosion of stream banks, changing the landscape faster than we might realize, and damaging freshwater ecosystems downstream. This is especially true for streambanks that lack woody plants to hold the soil in place.

Trees, shrubs, and native grasses help keep soil intact! One way to reduce sediment is to protect and maintain the plants growing alongside our streams. Riparian buffers are the vegetated areas located along the banks of streams, rivers, wetlands, and lakes. A healthy riparian buffer consists of native trees, shrubs, and grasses that act as a natural barrier between land and water. Riparian buffers play a critical role in protecting water quality by filtering pollutants, reducing erosion, and preventing runoff from flowing into our waterways. Additionally, they provide wildlife habitat, improve flood control, stabilize streambanks, moderate water temperatures (trees provide shade), and enhance climate resilience.

• Gilt Darter – The Gilt Darter requires cool, fast-flowing streams with clean gravel or rubble riffles and aquatic plants to host its prey: aquatic insects like mayflies. Grab a mask and look for them in upper French Broad streams!

• Eastern Hellbender – The Eastern Hellbender is one of only three giant salamanders found in the world. Hellbenders breathe through their skin, making them susceptible to sediment and other pollutants in their aquatic habitat. Hellbenders shelter in the spaces underneath rocks and when excess sedimentation fills in these spaces, they lose this vital habitat.