Sacramento River Gradient Restoration Facility

The Glenn-Colusa Irrigation District (GCID) pumping station is the largest diversion on the Sacramento River in Northern California. Water has been diverted for irrigation since roughly the turn of the century, and protection of fish species has historically been a concern, as some type of screening has been associated with the diversion facility since the early 1920s. Sustained high flows in 1970 caused the cutoff of a meander bend downstream from the pumping station, reducing the river length by over 2.4 km (1.5 miles). As a result, water surface elevations near the fish screens have dropped approximately one meter, generating higher velocities through the screens and causing increased mortality rates of juvenile salmon from impingement on the screens. Since the cutoff occurred, several fish screening alternatives have been incorporated into the facility to reduce the loss of fish. However, these attempts have proven to be less than satisfactory, resulting in mandated pumping restrictions since 1991. A unique hydraulic structure was designed in the main river to raise water surface elevations under low flow conditions, while still maintaining adequate upstream fish passage and navigation. This structure, or gradient facility (GF), is one component included in a comprehensive fish screen improvement project and was designed to raise upstream water surface elevations while mimicking hydraulic conditions found in natural riffles within the Upper Sacramento River, and thereby accommodate both upstream fish migration and navigation. This paper provides an overview of the extensive engineering analyses and design methodologies adopted to formulate this unique structure.