Located southwest of Klamath Falls, Oregon, Klamath Straits Drain is a 10.1-mile-long canal that conveys water uphill and northward through the use of pumps before discharging to the Klamath River. Klamath Straits Drain traverses an area that historically encompassed Lower Klamath Lake. Currently, the Drain receives water from farmland and from parts of the Lower Klamath Lake National Wildlife Refuge. To support water-quality improvement in Klamath Straits Drain, a hydrodynamic and water-temperature model was constructed and calibrated for calendar years 2012–15 with the two-dimensional model CE-QUAL-W2 (version 4.0). Water quality was calibrated for a subset of that time, from April 1, 2012 to March 31, 2015. Flows in calendar year 2012 were within the normal range, while calendar years 2013–15 were dry years. Significant findings from this study include:

• In the years studied, only limited flow entered Klamath Straits Drain at the upstream Headworks (KSDH) site. Most flow entered the Drain between KSDH and the E-EE pumps near Township Road through several irrigation channels and ditches. Few data were available to describe the quality of this water for the period of study.
• The E-EE and F-FF pumps along Klamath Straits Drain mainly operated automatically to keep water levels relatively steady. Ten-minute flow data at streamgage 11509340, downstream of the F-FF pumps, showed high-frequency on/off switching of the F-FF pumps. Combined with daily mean flow data from the F-FF pumps, the downstream 10-minute flow data allowed estimation of 10-minute pumping rates for the F-FF pumps. Paper pump charts showed the existence of short-term variability at the E-EE pumps; however, daily pump data were used at the E-EE pump location in the model.
• Water temperature in Klamath Straits Drain varied from less than 5 degrees Celsius (°C) (with occasional ice cover in December–January) to greater than 20 °C in May–September. In the years studied, specific conductance was typically 250–850 microsiemens per centimeter, higher than Klamath River specific conductance (typically 100–200 microsiemens per centimeter).
• Increased chlorophyll a in autumn and winter, along with supersaturated oxygen concentrations, indicated algal blooms in the Drain at that time of year. The blooms were most likely diatoms, based on the timing of blooms sampled elsewhere.
• Total nitrogen concentration was as much as 5.5 mg/L, with most in dissolved organic and particulate forms, and lower amounts in ammonia and nitrate+nitrite. Total phosphorus concentrations were distributed between orthophophorus (at a median concentration of 0.15 mg/L) and organic and particulate forms (at a median concentration of 0.13 mg/L). Most of the organic carbon in the Klamath Straits Drain was in dissolved rather than particulate form.
• Newly collected water-quality data for April 1, 2012–March 31, 2015 helped provide the impetus for this modeling study. However, a lack of some data still hindered the construction and calibration of this model. The model would benefit from additional data to describe water-quality boundary conditions, water-quality calibration data upstream of the F-FF pumps, short-term E-EE pump operations, and channel bathymetry in the reach between Highway 97 and the confluence with the Klamath River.
• Klamath River water mixed upstream into the Klamath Straits Drain, up to the Klamath Straits Drain F-FF pumps at Highway 97, when the F-FF pumps were not operating for periods of hours to days. The F-FF pumps were off for many days during this study, especially during dry years.
• The boundary between Klamath Straits Drain and the Klamath River was best modeled with an external head condition, which allows exchange of water between the river and the drain in both directions, upstream and downstream.
• Currently there is a flow gage, water-quality monitor, and a water-quality sampling site located downstream of the F-FF pumps, in the reach where Klamath Straits Drain water can mix with Klamath River water. To sample solely Klamath Straits Drain water, water samples would need to be collected only when the F-FF pumps are actively pumping. Alternately, the sampling location could be moved upstream of the pumps. Interpretation and use of historical water-quality data at the Klamath Straits Drain at Highway 97 site should be done in conjunction with information on pump activity to help inform whether mixing with Klamath River water may have occurred.
• Total 2014 (a dry year) phosphorus loads from the Drain to the Klamath River were lower and closer to total maximum daily load (TMDL) allocations, as compared to 2013, a year with greater flow and pumping.
• Modeled travel time through the Klamath Straits Drain, from Headworks to its confluence with the Klamath River, ranged from approximately 24 hours at high flow to 16 days or more, depending on how many days the pumps were turned off. The longer travel times are sufficient for important water-quality transformations, such as algal growth and organic-matter decomposition.

This newly constructed model of the Klamath Straits Drain simulates flow, water levels, water temperature, and water quality with acceptable accuracy but with certain data limitations. This model should prove useful in evaluating potential strategies for flow and water-quality management and restoration.