What is meant by water quality?

The water in a stream does many things. It is a home for aquatic plants and animals; birds and other land animals rely on it; people drink it; people fish and swim in it. A stream with high water quality is able to do all these things well; as water quality deteriorates, so does the stream’s ability to fulfill these functions.

The North Carolina Division of Water Quality (NCDWQ) determines the water quality in a stream by counting aquatic macro-invertebrates – animals without backbones, such as insects and worms, that can be seen without a microscope. A healthy stream will have a lot of macroinvertebrates, of many different species living in it. If there aren’t many bugs, and/or if the ones that are there are from the few species that aren’t very sensitive to pollutants, then the stream is considered impaired. Ellerbe Creek has been classified as impaired.

Why does our creek have water quality problems?

Many different things can contribute to decreased water quality. There are two main categories: stuff in the water, and alterations in how fast water gets to the stream.

Our streams are a lot cleaner these days than they used to be because laws prohibit dumping of pollutants into them. Most of the unwanted stuff in the water comes from non-point sources – stuff washed off of roads and other surfaces by rainwater, and sediment eroded from stream banks and off of exposed soil at construction sites. NCDWQ considers sediment to be the leading cause of water quality impairment in North Carolina's streams and rivers, while nutrients (nitrogen and phosphorous) are the most significant problem in our lakes. Regulation of stream water quality is largely focused on reducing the amount of nutrients flowing into lakes. This is good for the lakes, but doesn’t necessarily help the streams

Often overlooked by regulators is the impact of changes in stream flow volume related to stormwater runoff from impervious surfaces - things that water can’t pass through, such as pavement, sidewalks, and roofs. Most of the rain falling within the watershed runs off into storm drains and ditches, and is dumped quickly into streams instead of soaking into the ground and traveling slowly to the streams, particularly in areas with a lot of impervious surfaces. Groundwater is the main source for water in streams between storms, and so urban streams often have very little water in them when it isn’t raining.

The Ellerbe Creek watershed is mostly urban, with a high percentage of impervious surfaces. During storms, too much water gets to the Creek too rapidly, causing scouring of the streambed and erosion of the stream banks, and may cause flash floods. The water flow drops nearly as rapidly after the storm passes. So much of the rain flows down the stream so quickly that the stream level is unnaturally low in between storms. This pattern of extreme low and high flows due to impervious surfaces and storm drains makes it very difficult for animals and plants to survive.

As any gardener knows, Durham’s soils are unusually clay-rich. Water has trouble sinking into such clay-rich soils unless those soils have been opened up by deeply-rooted plants like trees. Almost all of the rain that falls on a forest sinks into the ground or returns to the atmosphere as water vapor. Grass has very shallow roots, and doesn’t open up the soil very much. Therefore, although lawns hold more water than pavement, much more rainwater tends to run off of lawns than from forests.

Why are we monitoring stream flow?

Stream flow, or discharge, is the volume of water that moves past a point in the stream over a given period of time. It is usually reported in units of cubic feet per second (ft³/sec). As in other urban streams, discharge varies enormously over time in Ellerbe Creek in response to storms.

The extreme high and low stream flows that are caused by impervious surface runoff do tremendous damage to Ellerbe Creek, and make it very difficult for animals and plants to live in it. To improve conditions, we need to learn more about stormwater flows in Durham, particularly about how different combinations of impervious surface, lawns, and forests affect that flow, given Durham’s clay-rich soils. This type of information is necessary in order to design effective stormwater management projects.

The U.S. Geological Survey monitors water flow in Ellerbe Creek near Gorman NC, just before the Creek reaches Falls Lake. Although extremely useful, this single monitoring station doesn’t provide quite enough information to let us figure out best management practices. The City of Durham doesn’t collect any additional stream flow data.

To help fill this information gap, ECWA has established two volunteer flow-monitoring stations. One is at the east end of the 17-Acre Wood trail, across from Westover Park; the second is near the corner of Washington St. and Club Blvd., at the west end of the new greenway trail that parallels I-85. Both are small tributaries that drain relatively small areas (sub-basins) within the watershed; the sub-basin draining to the 17-Acre Wood site has a lot of grass and forest, while the sub-basin draining to the Washington St. site is mostly urban residential. Comparing differences in how the two small tributaries respond to storms with the differences in land cover between the two sub-basins will give us the information we need for planning.

Data collected from both sites will be posted here.

 

RELATED LINKS

• U.S. Geological Survey Real-Time Water Flow Data, Ellerbe Creek near Gorman, NC (close to Falls Lake) - waterdata.usgs.gov/usa/nwis/uv?site_no=02086849
• GoMap, Interactive GIS for Durham (City and County) - gisweb.durhamnc.gov/gomaps/map/index.cfm
• EPA Water Science homepage - www.epa.gov/waterscience/
• EPA Monitoring And Assessing Water Quality homepage - www.epa.gov/owow/monitoring/
• Durham’s Stormwater Services Division - www.durhamnc.gov/departments/works/stormwater.cfm