Scotts Creek Nutrient Erosion Study, Lake County, California

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Clear Lake, the largest natural lake entirely within California, has a severe problem with harmful algal blooms which can be detrimental to aquatic life by depriving it of oxygen when the algae dies, sinks, and decays. A driving factor of these conditions are the nutrient loads being carried into the lake by soil erosion and the transport of sediment from the lake’s tributaries. Recently, an increase in wildfire size and frequency may have led to additional nutrients being introduced into the lake.

Mount Konocti on the western shore of Clear Lake, California

Aerial  view of the western shore of Clear Lake, California.

One area of concern is the South Cow Mountain Recreation Area which includes the South Cow Mountain Off-Highway Vehicle (OHV) Management Area, in the headwaters of Scotts Creek, which flows into Clear Lake.  Sediment and dissolved nutrients may flow from South Cow Mountain into the lake via Scotts Creek. The area was severely burned during 2018 in the Mendocino Complex Fire. In this study, USGS scientists will make a baseline assessment of Scotts Creek and its tributaries to know where erosion is occurring. Results from this research will help the federal Bureau of Land Management (BLM) evaluate the cause(s) of the erosion and how erosion contributes nutrient loads going into Clear Lake. In addition, the results will be used by the BLM to establish a long-term monitoring program to demonstrate possible reductions in the nutrient loads coming from Scotts Creek once erosion control measures have been implemented.


  1. Better understand the impact of wildfire on soil erosion and resulting transport of nutrients in the aquatic environment;
  2. Provide an opportunity for USGS to assist BLM in studying erosion from an active OHV park that has been affected by a recent wildfire. Specific objectives are to: a) quantify erosion rates in the South Cow Mountain area; b) quantify nutrient loads coming from the South Cow Mountain area using water-quality sampling at gaging stations;
  3. Determine nutrient sources in the Scotts Creek drainage; and
  4. Construct rainfall-runoff models of the Scotts Creek watershed with sediment and nutrient transport.

Scientific Approach

The proposed work is divided into four tasks, each addressing one of the study objectives:

  1. Erosion Rate from Aerial Coverages;
  2. Nutrient Monitoring and Loads;
  3. Sediment Fingerprinting; and
  4. Watershed Modeling.
Scotts Creek sampling sites near Clear Lake, California

Aerial photo showing sampling sites 1 and 2 and two new USGS stream-gaging stations that have been installed. (Click to enlarge.)

Task 1 will use data from airborne lidar along with digital surface models derived from stereo aerial photographs using Structure from Motion methods to quantify long-term erosion rates on South Cow Mountain. GIS analysis will determine if there is a spatial relationship between erosion rate and OHV activity.

Task 2 includes water-quality sampling at four locations over five water years in the Scotts Creek drainage. Annual nutrient loads will be computed at the sampling sites where continuous flow records are available. Two new USGS stream-gaging stations have been installed for this project (see photo):

Task 3 involves sediment fingerprinting focused on determining nutrient sources in the Scotts Creek drainage. Samples of soils, streambed sediments, and streambank deposits representing the range of land use/land cover, soil types, and burn severity will be collected and analyzed for a suite of constituents.

Task 4 will characterize the relations among flow, transport of sediment, and particulate nutrients using a watershed-scale computer model which will provide a useful management tool for BLM to evaluate potential best management practices for mitigation of erosion from the OHV area.

Relevance and Benefits

This study will address the USGS water science strategic objective of advancing monitoring networks and techniques for determining water quality.

The study will also generate data on nutrient loading that can be compared to other models for the watersheds in northern California that have been affected by wildfire. This will help verify model results and also inform future modeling efforts of potential effects of wildfire and fire retardants on nutrient loads.

Wildfire is mentioned numerous times in the USGS hazards science strategy. The data collected in this study could contribute to the Landscape Fire and Resource Management Planning Tools (LANDFIRE) an interagency vegetation, fire, and fuel characteristics mapping program sponsored by the U.S. Departments of the Interior and Agriculture and executed by USGS and the U.S. Forest Service (USFS).

This study will produce data of relevance to the USGS Environmental Health mission area, which also mentions wildfire numerous times in its science strategy

The data and models generated in this study will also be of interest to many stakeholders in the Clear Lake watershed who are concerned with the water quality of Clear Lake, including California state agencies, Lake County, other local agencies, several Native American tribes in the area, and the general public. Regular updates on study progress will be provided to the Blue Ribbon Committee for the Rehabilitation of Clear Lake, through its Technical Subcommittee.