Causes of Impairment in Ellerbe Creek
Ellerbe Creek is one of twenty-eight local watersheds that drain into the Falls Lake Reservoir. The Upper Neuse River Basin Association identified the Ellerbe Creek watershed as having the highest percentage of impervious surfaces and delivering the highest nutrient loads to the Falls Lake water supply reservoir.¹ Ellerbe Creek is rated as “not supporting” designated uses; in 1995, it was one of the four lowest rated streams in the Neuse River basin.² The main source of pollution in the Ellerbe Creek watershed appears to be nonpoint source pollution from urban and suburban areas, primarily stormwater runoff from urbanized lands.³

According to the State of North Carolina, stresses on streams fall into two broad classes: 1) chemical or physical pollutants, such as toxic chemicals or oxygen-consuming wastes; and 2) habitat degradation such as loss of riffles and pools due to sedimentation or scour due to increased force of waters from stormwater flow or wetlands loss. In North Carolina, the number one rated cause for streams to be rated as impaired is poor biological integrity, which means that the stream does not have an appropriate number and/or variety of benthic macroinvertebrates. These animals, primarily aquatic insect larvae, are sensitive to subtle changes in water quality and can be indicators of a wide range of environmental stresses.⁴

The main source of stream water impairment is runoff from impervious surfaces in urban and suburban areas. These surfaces alter the rate at which stormwater is delivered to streams. Rather than soaking into the soil and becoming groundwater, precipitation flows quickly off of impervious surfaces into stormwater conveyance systems that dump into streams. As a result, storms produce extreme high flows and flash floods that scour the stream channels, flushing sand and gravel in the stream bed downstream, and eroding stream banks, adding fine sediment to the water column. Both the scouring and the sediment loading are extremely harmful to the animals and plants living in the stream. Like other streams in the region, Ellerbe Creek tends to experience extreme low flow conditions during the dry, hot summer season. These extreme low flows limit the streams’ ability to assimilate oxygen-consuming wastes from stormwater runoff and wastewater treatment plants,⁵ thus contributing to the problem of nutrient loading in the reservoir lakes. The extreme low flows also have direct negative impacts on the biological integrity of the streams themselves.

Stormwater runoff carries natural and human-made pollutants into waterbodies from a variety of sources and may include sediment, nutrients, bacteria, oil and grease, trace metals, toxic and synthetic chemicals.⁶ Chemical monitoring overseen by the State of North Carolina provides evidence for the sources of pollutants entering Ellerbe Creek. The monitoring has shown that improvements at Durham’s waste water treatment plant over the last decade have been effective, and the plant is not a major contributor to nutrient loading.⁷ Dissolved copper and zinc are still consistently above state standards, providing evidence that stormwater runoff from roads and parking lots is a significant contributor of pollutants to the stream. As noted by Lee and colleagues (2007), "Industrial and transportation land uses are generally known as greater sources of heavy metals (generally six metals: cadmium, copper, chromium, lead, nickel, and zinc) than other land uses such as residential land use… Industrial land use has the highest median concentrations for cadmium, chromium, lead, and nickel, but copper and zinc are the highest from highways."

The negative impacts of stormwater runoff on Ellerbe Creek are exacerbated by the non-natural geometry of the stream channel. In the 1950s and 1960s, the natural meander and habitat of Ellerbe Creek’s channel was obliterated when the Army Corps of Engineers (USACE) straightened and deepened the creek. This channel modification, intended to reduce the risk of flooding, lead to greatly increased erosion of the creekbanks and an “abandonment” of the floodplains. “In a healthy stream, floodplains are a place where floodwater can disperse its energy, drop its sediment, and percolate into the groundwater. A deeply cut stream channel prevents these beneficial processes from occurring, as floodwater instead is carried in a powerful torrent downstream, where it can actually increase flooding in downstream areas.”⁸ ECWA was alerted that USACE flood control measures for channel maintenance that were put into place when the stream was straightened are still being followed by Durham’s Public Works department; ECWA is working with the city and state to put an end to these outdated practices. These measures include removal of any trees that are leaning and might fall into the stream. The USACE is concerned about the potential of such trees to block stream flow and cause localized flooding. However, modern stream management practice discourages this practice, which damages the riparian zone (stream bank and adjacent land) and greatly reduces the amount of “coarse woody debris” in the stream, which is an important component of the stream ecosystem.

Water Quality Management Issues in Durham
Durham is dealing with two water quality issues that are functionally related, but that differ significantly in their impacts and in the appropriate management strategies for dealing with them. The first issue is the need to maintain the quality of the water in Falls Lake and Jordan Lake. This water is a major source of drinking water for the region, and therefore the main focus of NC DENR’s water-related actions in this region is on maintaining an adequate supply of water that can be cost-effectively treated to drinking water standards. NC DENR has determined that the largest (but by no means the only) source of water quality impairment in the lakes is nutrient loading – these lakes are considered to be "nutrient sensitive waters."

The second water quality issue facing Durham is impairment of its streams and rivers. According to Durham’s Public Works Department⁹, “[m]any streams and other waters in Durham County fail to meet water quality standards or to fully support all their designated uses. Two sources of pollution are identified: illicit discharges and illegal disposal, and stormwater runoff.” The second source is particularly relevant to Ellerbe Creek. According to the Ellerbe Creek Local Watershed Plan, “Ellerbe Creek is impaired because its natural flow or its hydrology (in essence, the way a stream works) has been dramatically altered.” Goal 1 of this plan is to “Improve Aquatic Life in Ellerbe Creek.”

Water quality issues in both lakes and streams need to be addressed. Because physical and biochemical processes differ in streams and in lakes, each requires a different management strategy. However, it appears that water quality management in Durham is focused primarily on improving conditions in Falls Lake and Jordan Lake, with less attention paid to reducing impairment of stream systems, including Ellerbe Creek. This unbalanced focus undoubtedly reflects a combination of the lake-centered regulatory demands by NC DENR and limited availability of management resources.

The Ellerbe Creek watershed highly urbanized. As noted above, stormwater runoff from impervious surfaces is the main source of stream impairment in suburban and urban areas in North Carolina, and thus any water quality management effort should emphasize stormwater management. As seen in Figure 1 and Table 1, connected impervious surfaces are most widespread in the south-central part of the watershed (downtown Durham), are less abundant in the western (headwater) end of the watershed, and are least abundant in the eastern end (near Falls Lake). The current plan, as explained at a public meeting in November 2008, is to focus efforts on conserving naturally vegetated lands in the relatively undeveloped eastern and western margins of the watershed. Although this is important for keeping the problem from getting worse, it does little to reduce the existing impairment. Improving existing conditions will require reducing the harmful impacts of impervious surface runoff from Durham’s urban core and the surrounding urban residential neighborhoods.

A Strategy for Stormwater Management in an Urbanized Watershed
For the purposes of stormwater management, the Ellerbe Creek watershed can be divided into three basic categories of subwatershed: low-impervious, medium-impervious, and high-impervious. Low-impervious subwatersheds are rural to suburban, with less than 25% connected impervious surface.¹⁰ Subwatersheds with less than 10% impervious surface are particularly valuable. Medium-impervious subwatersheds are dominantly urban residential, with some commercial and industrial properties. High-impervious sub-watersheds are dominated by commercial and industrial, along with high-density residential areas. These three different categories of subwatersheds require different stormwater management strategies.

• Low-Impervious Subwatersheds: The critical management activity in low-impervious watersheds is conservation of undeveloped land in order to limit increase in connected impervious surface area and to preserve riparian vegetation. This conservation work is being conducted by both government agencies and non-governmental organizations; ECWA is working to protect riparian land in this area through it’s partnership with the Upper Neuse Clean Water Initiative; a multi-party effort to protect lands rated critical to protecting drinking water supplies throughout the Upper Neuse River Basin. It is important to note that this conservation activity limits additional degradation of streams and lakes and does not result in reduction of existing impairment.

• Medium-Impervious Subwatersheds: The majority of the impervious surfaces in these subwatersheds consist of individual structures (houses) surrounded by vegetation (lawns and forests) and connected by roads and driveways. Parcels are relatively large. Stormwater runoff causes stream impairment, but the stream is not totally degraded; modest improvements in water quality and flow characteristics can result in an appreciable improvement in stream condition. The most effective approach to stormwater management in such subwatersheds is to treat the stormwater runoff from individual parcels by installing small BMPs, such as rain gardens and vegetated swales, on each parcel. These small BMPs are relatively inexpensive and can be very attractive. Unfortunately, few residents are aware of the need, and few landscape contractors or garden centers are capable of providing information and services.

  The Durham County Soil and Water Conservation District (SWCD) is addressing this capacity gap through their new Community Conservation Assistance Program (CCAP). The CCAP is a voluntary program that assists landowners to install stormwater BMPs on their property. Applicable only to existing development, the CCAP will reimburse up to 75% of the average cost of the installation of qualifying BMP’s. SWCD staff provide information about BMP design. This excellent program has only been in place for one year, and has limited resources, and so has not yet had an appreciable impact. A serious challenge for such distributed treatment programs is that a large number of the small BMPs, adding up to a threshold volume of stormwater, must be in place before significant benefits are realized in the form of water quality and stream condition improvements. The CCAP is, of necessity, starting slowly, and the limited available resources means that there is a very real danger that it will not succeed in achieving a threshold level of treatment. However, the program design is highly appropriate for medium-impervious subwatersheds, and thus it should be fully supported by Durham’s government and citizens, and by the State of North Carolina.

• High-Impervious Subwatersheds: The biggest water quality management challenge facing Durham is mitigating the negative impacts of the high volumes of stormwater runoff in high-impervious subwatersheds. Parcels are very small and generally lack sufficient pervious surface area to handle the large volume of stormwater runoff from that parcel. In the few instances where cities have attempted to deal with stormwater runoff from high-impervious surface subwatersheds, the focus has been on constructing conventional structural BMPs capable of handling large volumes of stormwater runoff, such as extended detention dry ponds, wet ponds, and infiltration basins. However, such structures are often impractical to implement in such settings.¹¹ Only a very limited amount of open space (undeveloped land) is available for stormwater treatment, and there are many competing demands for that limited space, notably recreational uses such as ballfields.

  Decentralized stormwater management has been suggested as an innovative and cost-effective alternative to large structural BMPs in urban settings (Keeley 2007). The CCAP strategy for medium-impervious sub-watersheds is an example of a decentralized management system. The strategy is somewhat different in a high-impervious subwatershed, in that the small BMPs are not tied to particular parcels. Instead, many small, relatively inexpensive BMPs are put in place wherever possible, each handling only a small amount of the total stormwater runoff. Even small depressions around individual street trees can contribute to the total volume treated. Many, if not most, of these small BMPs are likely to be on publicly maintained land, including sidewalk easements and other infrastructure-related corridors.

  The threshold problem described for medium-impervious subwatersheds is even more severe in high-impervious watersheds. This problem can best be overcome by installation of one or more medium-sized BMPs. Although not large enough to mitigate stormwater impacts on their own, such projects can produce a measurable improvement in stream conditions. A decentralized stormwater management strategy that includes one or more medium-sized BMPs can reach significant water quality goals with a practically achievable number of small BMPs.

Stream Restoration Projects as Part of a Comprehensive Approach
Stream restoration projects have become common in urbanized areas. Unfortunately, such projects often treat symptoms (incised channel morphology along a short stream segment) rather than underlying causes (extreme low and high flows associated with runoff from impervious surfaces), are little more than temporary, aesthetically pleasing band-aids. Stream restoration projects in the absence of “naturalized flow” will almost always have only limited and temporary positive impact on stream conditions. Stream flow shapes stream channels; if the flow characteristics change, the channel morphology will change accordingly. We can speed this process up by mechanically reshaping the cross-section of the stream channel; however, unless extreme care is taken with design and execution, mechanical reshaping results in soil compaction and disruption/removal of the organic soil layer and its irreplaceable load of native plant seeds and essential microorganisms. Thus, while well-designed stream restoration projects that are coupled with stormwater management can improve stream conditions, many restoration projects may prove utterly devastating for native plants and thus for valued stream and riparian ecosystems. In addition, in the absence of improved stormwater managemnt, restorations may be severely damaged by extreme high flows. Given these factors, it is essential that stream restoration not be viewed as a panacea but instead as one component in a comprehensive management approach that prioritizes underlying causes and protects native plants and microorganisms.