Nutrient and Sediment Loading to Upper Klamath Lake

As rivers and streams move across the landscape, they transport more than just water. Eroded sediments are transported downstream until the water loses enough energy to deposit them. Sediment can release phosphorous in many forms, including dissolved phosphorus. So, when sediment enters a water body, it contributes to the total phosphorous load; and dissolved phosphorous is a primary nutrient for algae and plants. 
 

Suspended sediment and total phosphorous loads are not easy to directly measure. At the USGS our scientists use statistical models to compute concentrations of sediment and phosphorus entering Upper Klamath Lake from the Williamson and Sprague Rivers. The models combine data from physically collected sediment and phosphorus samples into a calculation that uses streamflow and turbidity time-series data to calculate sediment and phosphorous loads going downstream and into Upper Klamath Lake.  
 

Why It Matters 

Upper Klamath Lake (UKL) is important for people, wildlife, and the environment. It’s home to endangered fish like the Lost River and Shortnose suckers, which are culturally important to the Klamath Tribes. The UKL is naturally rich in nutrients (eutrophic) but has become hypereutrophic during the previous 100 years. Nutrient levels have been high enough to cause annual, extensive blue-green algae blooms each summer.  Problems caused by the life cycle of algae blooms include foul odor along shores and bays during bloom die-offs, high pH, fluctuating dissolved oxygen levels including anoxia, and elevated ammonia concentrations. Poor water quality is one of several factors presumed to effect poor survival of juvenile Shortnose and Lost River sucker. 
 

The Oregon Department of Environmental Quality has a total maximum daily load (TMDL) goal to reduce phosphorus entering the lake. The TMDL targets a 40 percent reduction in total phosphorous loads to help moderate the dense algae blooms that occur each summer and improve water quality. But natural disasters like wildfire can set back that progress.  
 

Wildfires can drastically change how water flows through the landscape—washing ash, soil, and nutrients like phosphorus into rivers and lakes. 
 

In 2021, the Bootleg Fire burned over 400,000 acres in southern Oregon. A large portion of that burned land drains into UKL. We're working to understand how that fire has changed the amount of sediment and nutrients entering the lake. 
 

Our research helps show whether wildfires like the Bootleg are making it harder to meet water quality goals—and what can be done about it. 
 

What We’re Doing 

We’ve been monitoring water quality in two rivers that feed into Upper Klamath Lake—the Sprague and Williamson Rivers—since 2008. These rivers naturally carry a lot of the lake’s phosphorus and sediment. 
 

• Measuring water clarity (turbidity) every 15 minutes using automated sensors. 

• Collecting samples routinely and during high-flow storm events to test for total phosphorus, dissolved phosphorus, and suspended sediment concentration. Phosphorus and suspended sediment data are analyzed at the Klamath Tribes Sprague River Water Quality Lab.  

• Updating statistical models that estimate phosphorus and suspended sediment concentrations and loads based on turbidity to include longer-term watershed response and recovery following the Bootleg fire. 

• Quantify how wildfire, rain, streamflow, and vegetation all interact to affect the sediment and phosphorous loads entering UKL. 
   

How This Helps 

This long-term study gives scientists, indigenous Tribes, land managers, and the public a clearer picture of how wildfires affect water quality in the Klamath Basin. The information helps: 

• Track progress toward meeting water quality targets. 

• Understand the timing and sources of phosphorus transport. 

• Plan restoration projects or other solutions. 

• Protect endangered fish and improve habitat. 
   

This research also supports the USGS mission to understand how water quality changes under pressures like climate, land use, and natural disasters.