2009 SAC water quality overlay of hydrology-based instream flow recommendations working draft

SB 3 directed the development of environmental flow recommendations for Texas waters through a science-based determination and stakeholder process, followed by Texas Commission on Environmental Quality (TCEQ) rulemaking. Environmental flow regimes are defined as schedules of flow quantities that reflect seasonal and yearly fluctuations for specific areas of watersheds, support a sound ecological environment, and maintain the productivity, extent, and persistence of key aquatic habitats. The Science Advisory Committee (SAC) provides an overview of how hydrologic data might be used to develop hydrology-based flow regime recommendations (SAC 2009a) and describes one piece of the collaborative process envisioned by SB 3 for the identification of flows to maintain a sound ecological environment in rivers and streams. The document notes that other disciplines such as biology, geomorphology, and water quality also warrant specific attention to ensure that instream flow recommendations are based on the broadest set of information available. The approach taken by the SAC is to have these disciplines addressed as separate assessments or overlays on the hydrology-based analyses. Water quality is the focus of this overlay document. Numeric and narrative criteria developed by the state address matter carried in suspension and solution, such as dissolved and suspended solids, as well as nutrients, toxics, indicator bacteria, temperature, pH, dissolved oxygen, and other parameters. Under some circumstances all might play a role in the determination of an environmental flow regime. Changes in a flow regime can be expected to produce changes in water quality conditions. The challenge is to ensure that the recommended flow regime protects water quality, particularly during low or subsistence flow conditions, and also considers water quality needs during higher flow conditions. It is often assumed that under natural conditions, which may have existed prior to human impacts, the quality of the water supports the desired sound ecological environment. While this may be true it should be recognized that it may be impossible to return to the naturalized flow condition due to land use changes, point and nonpoint source discharges, water supply needs and operational constraints. In addition, ecological changes may have already occurred in response to altered land use patterns and flow regimes. It must also be recognized that natural conditions encompass a substantial range in all of the dimensions of water quality in response to hydrologic, seasonal and weather variations and include a full range of outcomes. The Water Quality Overlay provides an overview of Texas water quality programs along with the programs that collect and make water quality data available. It then discusses various aspects of the relationship between water quality and subsistence flows and water quality and base and higher flow conditions. The document includes a specific example of the relationship between flow and various water quality parameters and provides steps the BBESTs can take to consider water quality issues in their recommended flow regimes.