Floodplain Restoration Can Reduce Stream Temperatures for Optimal Aquatic Health

Cold water streams provide excellent fish habitats. Unfortunately many streams in southeastern Pennsylvania run too warm to support fish hatcheries. Since most streams in agrarian areas lack sufficient vegetative buffer, tree planting programs are a common practice to stabilize stream banks and provide much needed shade to cool water temperatures. While tree planting is fairly easy and relatively inexpensive, the efficacy is limited. Great effort is required to protect the young saplings through the long time required to reach maturity. All too often a storm flood event washes away the saplings before the roots structure has had time to establish. The next season, a fresh round of tree planting starts anew.

But what if there’s a better way to return a stream to its natural state, resulting in consistently lower water temperatures, better vegetation and biological survival rates, and improved stormwater management? This is the question LandStudies, a Lancaster County firm dedicated to solving modern environmental issues with natural sustainable solutions, has been attempting to answer with their stream restoration projects.

A recent LandStudies project along Kurtz Run in Lancaster County serves as an example of the work they’ve been doing, which involves grading of the floodplain by removing legacy sediments which have been deposited in the original floodplain over decades, and many times, centuries. After grading, a new stream channel is established, natural in-stream structures for stabilization are installed, and wetlands are created. By comparing pre- and post-restoration daily maximum temperatures, LandStudies was able to determine that this method of floodplain restoration significantly reduced the effect of solar radiation on the temperature of the stream, much more-so than traditional tree planting “restorations.”

The reason this method of restoration is so successful in reducing stream temperature, is that by grading the floodplain and removing legacy sediments, the surface water of the stream is being reconnected to the cooler groundwater of the bedrock aquifer, below; whereas, in its pre-restoration state, the streambed was disconnected from the groundwater aquifer by sediments and debris. This enhanced surface-groundwater interaction is referred to as hyporheic exchange, and would have been how the stream operated naturally, before the land was settled and cleared for development and agriculture.

In addition to lowering water temperatures, this method broadens the floodplain, creating more gently-sloped surface area over which the stream can expand during heavy rain events, instead of rushing down a narrow channel. This reduces area flooding, erosion of the stream bank, and sedimentation in the stream, while increasing its capacity to take on rain and stormwater. The cooler, more stable stream characteristics are favorable to vegetation and other organisms, such as fish, allowing these ecosystems to better survive and thrive in the restored stream and floodplain.

Beyond its many ecological benefits, floodplain restoration projects provide communities with a way to address MS4 issues, protect drinking water supplies, recharge groundwater aquifers, and enhance green infrastructure.