Agenda 11-02-23; 4-a - Resolution Endorsing Consensus Principles II for Revised Falls Lake Rules

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Af� 20 Background and Supporting Information History of the Reservoir and Characteristics of Falls Lake and its Watershed The history of Falls Lake reservoir provides important context for its current water quality. The "lake" is a man-made body that results in an impoundment of the Neuse River at the natural fall line of the river at "the Falls of the Neuse." The placement of the Falls Lake Dam converted a natural, riverine environment to one with hydrologic, habitat, and ecological conditions very different from those that existed previously. This conversion has resulted in factors that affect water quality that were not present before the river was dammed. The reality of this history brings us to the regulatory and water quality policy crossroads of how this management effort should proceed over the coming decades. The Falls Lake reservoir project was authorized by Congress as part of the Flood Control Act in 1965 and began filling in January 1983. Figure 1 shows an old earthen dam uncovered during the construction of the Falls Lake dam in the late 1970s. The Congressionally authorized uses of Falls Lake include flood -- control, drinking water supply, recreation, fishing, aquatic life, and wildlife. Design and construction of the impoundment were conducted by the U.S. Army Corps of Engineers (USACE), which continues to manage and operate the reservoir today. Figure 1.Construction of the Falls Lake Dam,photograph courtesy Pre-impoundment studies predicted that Falls of the US Army Corps of Engineers as cited by the Wake Forest Historical Museum. Lake would be highly eutrophic (over-enriched with nutrients), especially in the shallow, upper end of the lake (DNER 1973, USACE 1974, NCDEM 1983). These studies also predicted that chlorophyll-a concentrations would be high and that dissolved oxygen would be depleted in deeper portions of the lake during thermally stratified (warm summer) conditions. Laboratory analysis of chlorophyll-a measures the green pigment in a water sample and is used as an indicator for algal growth. In 1979,the NC EMC established a chlorophyll-a criterion of 40 pg/L. The driving force for this water quality standard was poor water quality conditions on the Chowan River in northeastern NC. The Chowan River is a tributary to Albemarle Sound. Water quality conditions in this estuary in the 1970s resulted in demonstrated impairments to the waterbody that directly impacted the classified, designated uses. In response to these impacts,the Division of Environmental Management(predecessor agency of what is now the Division of Water Resources, DWR) evaluated these conditions and convened a group of established scientists and subject matter experts. This group provided a recommendation for a benchmark level of algal activity, using chlorophyll-a as indicator. This threshold would guide regulatory action to address the problems on the Chowan River. The agency, through the EMC, adopted the benchmark level as a standard and applied this value to all waterbodies in the state. The evaluation that led to this standard was not specifically referenced to use support impacts in reservoirs. Despite the water quality predictions provided in the pre-impoundment studies, it was concluded that the expected levels of algal activity would not impede the anticipated designated uses of Falls Lake. NC allowed the construction of the dam under Section 401 of the Clean Water Act (the Water 1