DEVELOPMENT OF A HYDRODYNAMIC AND WATER QUALITY MODEL FOR A HYDRO-MODIFIED INTERSTATE SYSTEM TO SUPPORT TMDL DEVELOPMENT: LOST RIVER, OR & CA

The Oregon DEQ (ODEQ) and the North Coast Regional Water Quality Control Board (NCRWQCB) have both included the Lost River, a tributary to the Klamath River, on their Clean Water Act (CWA) Section 303(d) lists of impaired waters as a result of observed water quality criteria exceedances. Impairments include dissolved oxygen (DO), chlorophyll a, nutrients, temperature, fecal coliform, and pH. ODEQ and the NCRWQCB are separately responsible for developing and submitting TMDLs for their respective waterbodies. A fundamental element of the TMDLs is the development of a hydrodynamic and water quality model that can assist in determining loading capacities, load allocations, and waste load allocations. Tetra Tech has been working very closely with ODEQ and NCRWQCB, as well as EPA Regions 9 and 10, to identify and apply a modeling approach to support Lost River TMDL development (Tetra Tech, 2005). Once the Lost River model has been completed, Tetra Tech will develop a comprehensive model for the Klamath River from Upper Klamath Lake to the Pacific Ocean. The Lost River model will be linked to the Klamath River model for TMDL development.The Lost River is a highly hydro-modified system that is predominantly fed by lake diversions from outside the natural watershed and reservoirs in the upper-watershed. It contains multiple impoundments along its length that cause the river to exhibit highly variable hydraulic conditions, including free-flowing riverine segments and relatively stagnant reservoirs/ponds. These conditions, along with significant return flows and withdrawals from adjacent agricultural lands, lead to significant water quality and flow variability within short time periods. These dynamics have a dramatic effect on water quality and lead to a unique, biologically active system that is inundated with phytoplankton and macrophytes during the spring, summer, and fall. To support TMDL development for the Lost River system, the need for an integrated receiving water hydrodynamic and water quality modeling system was identified. The proposed approach included implementing the U.S. Army Corps of Engineers' CE-QUAL-W2 (W2) model for the entire Lost River system from Malone Dam to the outlet of the Klamath Straits Drain (feeding into the Klamath River).The first steps in the TMDL development process have already been conducted and included compilation of available data from the various agencies; evaluation of monitoring data to identify the extent, location, and timing of water quality impairments; and development of a technical approach to analyze the relationship between pollutant loading contributions and in-stream response. Because of the unique hydrodynamic features at different sections of the Lost River, the entire system was divided into 12 modeling sub-domains where each of the sub-domains were further divided into computational segments and layers to accommodate the finite difference solution scheme in W2. A number of enhancements to W2 version 3.2 were implemented to improve model efficiency and Sediment Oxygen Demand (SOD) and periphyton/macrophyte representation. The model was configured and calibrated for 1999 data. Key model outputs included flow, water surface elevation, temperature, dissolved oxygen (DO), nutrients (PO4, NOx, NH3), chlorophyll-a, and pH. To support model validation, a focused physical, chemical, and biological monitoring effort was conducted during the summer of 2004.Although significant data limitations posed a challenge to model development and application, the approach implemented proved successful in deriving dominant boundary conditions and making relatively accurate in-stream hydrodynamic and water quality predictions to support TMDL analysis and evaluation of water quality standards. The modeling study suggests that macrophytes are the dominant factor controlling the diel DO and nutrient fluctuation, and specifically minimum DO levels. It also suggests many segments of the Lost River are limited more by nitrogen than by phosphorus, with respect to macrophyte development.