In preparation for the anticipated removal of four large dams from the Klamath River on the California coast, the USGS has begun a multi-year, intensive monitoring program to document the effects that dam removal will have on the river system. An estimated 10 million cubic meters of sediment currently trapped behind these reservoirs will be mobilized and transported along 313 kilometers of river between the dams and the Pacific Ocean. A baseline comprehensive river topographic survey incorporating both aerial light detection and ranging (lidar) and boat-based bathometric survey data was completed in 2018 to help document pre-dam removal conditions. This survey included the use of blue-green lidar, which can collect detailed measurements in shallow water areas that are typically excluded from traditional measurement techniques. It also provided a 1-meter, near-complete elevation model of the entire river corridor. Detailed elevation data on this scale allow for new evaluation techniques to quantify river geomorphons using statistical characteristics and can help quantify river form and function. On the Klamath River, one set of geomorphons the team is monitoring is the distribution of relative elevations as measured by the kurtosis and skewness at 100-meter intervals along the river. Kurtosis, as a measure of peakedness, can best be thought of as a proxy for river complexity. Sections of the river containing a dominant channel elevation will have a high kurtosis and are likely indicative of locations with few sediment depositional features and an armored or rock-limited channel bed. Low kurtosis areas, with a broad range of bed elevations evenly spread between deep and shallow areas, can be used to identify areas of more channel complexity with features such as pools, back eddies, or pointbars. Specific geomorphic features can be further resolved by examining the skewness of the elevation distribution, with pools creating a negative skewness while bar features can be identified by positive skewness. The benefit of using statistical measurements is that they are scale independent, which allows this technique to be used to monitor long river stretches and examine longitudinal shifts in river conditions at a range of watershed sizes. These measurements have been used to help identify representative sites along the river for on-the-ground monitoring and provide a useful baseline to monitor geomorphic changes in river conditions and lateral shifts in deposition patterns compared to pre-dam removal conditions.
A map showing the distribution of the kurtosis of relative channel elevations along the Klamath River. Inserted image shows two examples of elevation distributions. Top graph shows a high kurtosis section of river with simple channel geometry. Bottom graph shows a low kurtosis section of river with complex geometry.