The current channel of the lower American River is narrower, deeper, and artificially constrained by levees and dams compared to its channel prior to the Gold Rush. Prior to the Gold Rush, the American River frequently changed its course across the valley floor and once flowed through sections of present-day East Sacramento before being artificially redirected toward Sutter’s Landing.
During and immediately after the Gold Rush, the bed of the lower American River rose drastically due to a massive influx of mining debris.
Bed Elevation Rise:
In 1879, sediment storage calculations estimated that the lower American River’s channel bed was 5 to 30 feet (1.5 to 9 meters) higher than its pre-Gold Rush state.
The Cause:
This extreme rise, known as channel aggradation (bed raising), was fueled by hydraulic mining. During the Gold Rush era, hydraulic mining dumped massive volumes of gravel, sand, mud and cobble from the Sierra Nevada mountains directly into the river system.
Impact on the American River:
Because the riverbed filled with debris, the river channel lost its capacity to hold water. Normal winter flows suddenly caused catastrophic flooding. To protect the city from the rising water levels, the City of Sacramento undertook a massive engineering project in the 1860s and 1870s to raise its streets and buildings by up to 9.5 feet.
Following the outlawing of unrestricted hydraulic mining and the later construction of Folsom Dam, the river spent the 20th century cutting back down through this historical sediment. Today, the channel has largely eroded back down near its original, pre-Gold Rush baseline.
The American River Channel Today
The completion of Folsom Dam in 1955 as well as other flood control improvements once again has completely changed the dynamics of the American River. In addition, the modern river is now bound tightly by constructed flood control levees along much of its 30-mile urban stretch. Historically, seasonal high waters regularly overtopped natural banks, spreading across wide floodplains and low-lying basins.
Bed Incision and Sediment:
Following the construction of Folsom Dam and Nimbus Dam in the 1950s, sediment was trapped behind the barriers. Deprived of a natural supply of new gravel and sand from upstream, the modern river has actively eroded and lowered its own bed (incision) over the decades.
Fixed Alignment vs. Migration:
Historically, the river frequently changed its course across the valley floor and once flowed through sections of present-day East Sacramento before being artificially redirected toward Sutter’s Landing. Today, bank protection projects and levees lock the channel in place.
The Impact of Folsom and Nimbus Dam
Because Folsom Dam and Nimbus Dam completely block the natural flow of gravel, sand, and cobble from the Sierra Nevada mountains, the lower American River is now locked in a long-term sediment deficit.
As the river flows downstream, it retains its energy but has no incoming debris to carry. This creates “hungry water” that actively erodes the river’s own bed and banks. Over time, this dynamic will trigger several permanent changes to the future channel:
1. Continued Channel Incision (Bed Lowering)
Without a fresh supply of upstream gravel to replenish the riverbed, high-flow events will continue to scour and erode the channel bottom. Historical studies have tracked the riverbed lowering at rates up to 8.2 cm per year in some areas. In the future, the channel will continue to dig itself deeper into the valley floor, further disconnecting the river from what remains of its natural floodplain.
2. Armoring of the Riverbed
As the river scours the bed, it easily washes away finer materials like sand and small pebbles. Over time, only the largest, heaviest rocks and boulders are left behind. This process is called channel armoring. Eventually, the riverbed will become so heavily armored with coarse rock that the water will no longer be able to erode downward, effectively locking the channel depth in place but leaving a harsh, rocky floor.
3. Accelerated Lateral Erosion and Bank Failures
When a riverbed becomes heavily armored and cannot erode downward any further, the water’s energy shifts sideways. The future channel will experience increased lateral erosion, aggressively biting into its banks. Because much of the lower river is bordered by urban areas, this creates a perpetual battle for flood managers. Agencies like the U.S. Army Corps of Engineers must continuously install artificial bank protection—like rock rip-rap and deep seepage walls—to stop the river from washing away the levees.
4. Loss of Salmon Spawning Habitat
The sediment deficit has severe consequences for native wildlife. Chinook salmon and steelhead trout require loose, golf-ball-sized river gravel to build their nests (redds). As the existing gravel washes downstream into the Sacramento River and no new gravel replaces it, the spawning habitat naturally depletes.
Human Intervention: The Artificial Fix
To counteract this grim outlook, the future of the lower American River will rely on heavy human management:
Gravel Augmentation:
Local environmental coalitions like the Water Forum routinely dump thousands of tons of clean, sorted gravel directly into the river below Nimbus Dam. This gives salmon a place to spawn and gives the “hungry water” something to carry, intentionally slowing down the erosion of the natural channel.