Volunteer Monitoring Demonstrates Tree Plantings Help Stream Ecosystems

Dr. Greg Noe, U.S. Geological Survey; Meghan Fellows, Fairfax County Park Authority; and Joanna Cornell, NVSWCD

(Conservation Currents, Northern Virginia Soil and Water Conservation District, Winter 2007)

Local streams receive plenty of abuse. Hit with pollution, storm runoff, and deforestation, both next to the stream (called the “riparian zone”) and within their watershed, we know that stream ecosystems are not what they used to be. Stopping these stresses and improving stream health will take the efforts of every resident, business, and government agency within our county. Strong scientific evidence supports the benefits of stream restoration, including planting vegetation in the riparian zone. However, information about the health of streams following restoration would be needed to help prioritize the locations and designs of future planting efforts.

Fairfax County, the Fairfax County Park Authority, and the nonprofit Earth Sangha have worked with volunteers to revegetate nearly 30 stream riparian zones since April 2005. The U.S. Geological Survey and NVSWCD are now monitoring and evaluating the effectiveness of one of these riparian plantings in restoring stream health. With the help of something called an “iButton”, a tiny temperature monitoring device, we are evaluating the impact of riparian zone restoration on stream temperature at Lake Fairfax Park in Reston.

The riparian restoration planting we evaluated was located just below the dam at Lake Fairfax Park, in a formerly mowed area where the field was too wet for recreation and mowing equipment was getting stuck in the mud. After an enormous volunteer planting effort, the area around the stream is now flourishing with wildlife and native plants like ironweed and sycamore saplings. But stream health is more than having the right plants and wildlife – it is also about having the right range of water temperatures. Too hot, and the water doesn’t have enough oxygen to support fish and insects. Hot water also speeds up some chemical reactions and the breakdown of organic matter, which can lead to a lack of food, too many nutrients, and oxygen deficiency.

Soon after planting at Lake Fairfax Park, volunteers began monitoring stream-water temperature. iButtons make collecting data on water temperature easy: they are the size of a stack of four dimes, relatively inexpensive, automatically store temperature at programmed intervals (every 30 minutes for us) over long time periods, and can be downloaded with a Palm Pilot. About once a month, in hot, freezing, wet, windy, or occasionally mild weather, the volunteers wade into the stream to download data from the iButtons.

The stream at Lake Fairfax Park is a shallow and narrow headwater stream that starts at a seep where ground water emerges from a hillside and ends 200 yards later when it joins the larger Colvin Run. From upstream to downstream, we’ve installed iButtons at five locations within the stream: just downstream from the ground-water seep; two sites in the currently forested riparian zone; in the riparian planting area between the forest and mowed grass where a few small trees, shrubs and tall grass now grow; and lastly, in a completely open area where grass mowing continues. An iButton also hangs off a tree to compare water and air temperature. Despite a few data gaps due to ornery iButtons or losing an iButton in the muck, over 100,000 measurements now document the influence of our land management on water temperature.

What is the effect of riparian vegetation on stream temperature? It depends on the time of year. When the air is cold, the water temperature also gets cold. In winter, the water got colder and colder as it flowed downstream from the ground-water seep (which has a relatively constant temperature), through the vegetated riparian areas and into the unplanted open site. During the coldest weather, the stream at the open, unplanted site froze solid for days, which rarely happened at the planted site just upstream. Vegetation at the planted site likely helped block the frigid winds and protect the stream. Needless to say, a solidly frozen stream is not a good place for wildlife, especially those that live in water!

But, in the summer, when it is hot, the water temperature got really, really hot. Where there was no vegetation, the stream temperature got extremely hot in the middle of the day in June, July, and August compared to the planted and forested riparian sites. At times, the stream water at the open site heated above 90ºF – that was 18ºF hotter than at the planted site just upstream where tall grass, shrubs, and trees shade the water.

Stream temperature at the five iButton monitoring locations from September 2005 to November 2006. Temperature changes registered by the iButton in the open, mowed field are more extreme than those where vegetation is present. Air temperature is also shown to compare with stream temperature.

Trees and shrubs help shade streams from solar heating during hot weather and help insulate streams from freezing weather. The extremes of our weather can lead to ‘fire and ice’ in streams without vegetated riparian zones. This buffering effect of riparian plants on stream temperature is crucial for maintaining good stream ecosystem health. Of course, trees in the riparian zone also provide food and homes for wildlife and help maintain water quality. As the trees grow and the canopy fills in, we’ll continue to track stream temperature as evidence of the importance of maintaining and restoring vegetation along our streams.

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