TMDL Modeling Approach for a Hydro-Modified River Where Numeric and Natural Conditions Criteria Apply: Klamath River Case Study

Hydrodynamic and water quality conditions in many rivers throughout the country have been greatly modified due to the presence of dams. Consequently, water quality criteria are often not achieved within impoundments themselves and in riverine stretches downstream of hydraulic structures. Under the Clean Water Act, states are required to develop Total Maximum Daily Loads (TMDLs) to restore waters that do not achieve water quality standards. TMDL development is not a trivial matter in highly hydro-modified systems due to a host of potential factors, including water quality standards interpretation and technical challenges. This paper presents key considerations and a modeling approach to determine TMDLs for a hydro-modified river; the important and controversial Klamath River.The Klamath River Basin drains nearly 16,000 mi2 on the California-Oregon border. Headwaters originate from Upper Klamath Lake in the Cascade Mountains, and the river flows in a southwesterly direction toward its confluence with the Pacific Ocean. Downstream of the lake the river contains a series of dams used for hydropower generation and flood control. Various segments, both riverine and impounded, exhibit water quality impairments. Parameters of concern include dissolved oxygen, nutrients, chlorophyll a, temperature, and ammonia toxicity.A set of hydrodynamic and water quality models were adapted and applied to the river system. This modeling framework was calibrated and corroborated using available monitoring data along its length for a range of conditions, and it was applied in a series of scenarios to support a proposed TMDL. Scenarios followed a logical progression that enabled both numeric and natural conditions criteria for relevant parameters to be fully evaluated and used as the driver for allocation. In general, when estimated natural water quality does not meet numeric criteria, the natural conditions criteria supersede those numeric criteria.The scenarios were grouped into the following categories: existing conditions, natural conditions, and TMDL compliance (including analysis of dam impacts). The calibrated and corroborated model, which consisted of 1-D RMA riverine models, 2-D CE-QUAL-W2 impoundment models, and a 3-D estuarine EFDC model, provided the basis for the existing conditions scenario. In order to fully evaluate applicable water quality criteria, it was necessary to simulate natural conditions throughout the Klamath River. The natural conditions scenario simulated the river in the absence of all dams and required application of an entirely different modeling configuration than that for the existing conditions. Considerations associated with natural conditions model development included selection of the appropriate models to apply when no dams are present on the river, representation of a natural reef that was later altered and replaced by a flood control dam, consideration of the natural flow regime, designation of boundary conditions based on existing upstream and tributary TMDLs, estimation of natural groundwater concentrations, designation of appropriate boundary conditions for tributaries not having TMDLs, and removal of all anthropogenic point and nonpoint source contributions.To achieve compliance with water quality criteria and determine appropriate allocations, more than 30 scenarios were simulated. Scenarios generally built upon the natural conditions model since numeric criteria were largely unattainable in the system. Scenarios run in the absence of dams determined draft allocations for point and nonpoint sources that addressed: 1) temperature compliance in Oregon, 2) temperature compliance in California, 3) dissolved oxygen compliance in Oregon, and 4) dissolved oxygen compliance in California. Once these allocations were determined, the final scenarios determined the effect of the dams by adding the impoundments to the mainstem while fixing all other pollutant sources at TMDL-compliant conditions determined from earlier scenarios. A range of considerations were addressed through these model runs including: criteria interpretation with respect to temporal impacts of different sources, changes in stream geometry between natural and current conditions, and changes related to water regulation and dam operations; allowance of anthropogenic contributions above natural conditions (for point sources, nonpoint sources, and dams); propagation of upstream water quality impacts to a location far downstream; and variability of periphyton and phytoplankton growth under different physical configurations.