Every October, a narrow stretch of water near Lodi turns silver with fish.
They arrive by the thousands, pushing up a fish ladder at a small irrigation dam.
Nobody built this river to matter much.
It barely shows up on a map of California’s water supply.
Yet something about it keeps pulling salmon home in outsized numbers.
The question is how a river this small ended up carrying so much weight.
Cold water releases timed each fall help set the size of the run
The Mokelumne River starts in the Sierra Nevada foothills and ends in the Sacramento-San Joaquin Delta, and by the numbers it should barely register. The utility that manages the river reports that the Mokelumne contributes only about 3 percent of the freshwater flow into the delta, yet its salmon population makes up as much as 50 percent of the commercial catch off the California coast. That gap between water given and fish produced is the whole puzzle of this river.
So what sits underneath that number? Below Camanche Dam, a hatchery and a stretch of enhanced spawning gravel do work that a bigger river would spread across hundreds of miles. Managers hold back a cold water pool in the reservoir and release fall pulse flows to draw returning fish upstream.
Fishing closures and decades of habitat work are also credited with lifting recent returns. None of it would show up in the record without the ladder itself, a stepped channel at the Woodbridge Irrigation District’s diversion dam near Lodi, where returning fish pass a video monitoring window.
One narrow ladder near Lodi funnels every returning fish past a camera
Picture a short run of fast water squeezed into a concrete channel, with salmon pushing against the current in bursts. That channel is the counting apparatus for one of California’s most productive salmon rivers.
Because every fish has to cross that single point, biologists get something rare in river science, an exact tally rather than an estimate built from sampling.
The same narrow plumbing is why the river’s fortunes can be read almost as they happen. A strong October shows up in the count within days, and a weak one shows up just as fast.
Pulse flows stepped from 330 to 1,330 cubic feet per second show the fine tuning
Each fall the operator modifies releases from Camanche Reservoir to create flood control space for winter and to attract Chinook, producing higher flows of cold, swift moving water. This October the published schedule called for an increase from 330 to 830 cubic feet per second in 100 cfs steps across an eight hour window, then from 830 to 1,330 the following day. That kind of fine tuning, repeated every autumn, is the unglamorous engineering behind a wild sounding number.
The counts swing hard from year to year. The 2023-24 run finished at 28,698 fish, beating a previous high of 19,953 set in 2017, and the 2024-25 run passed 30,000, the largest since record keeping began in 1940.
The following season came in at more than 10,500 fish, roughly a third of the record year but close to the river’s annual average of about 11,100 since 1998. Biologists also logged a single day record in that stretch, 3,824 fish over the dam on October 16, 2024, the largest one day count in more than 30 years.
Managers tie the softer season to water. “We came off of two years of record breaking runs, and we were expecting to loop that this year, but we fell way short of our expectations,” hatchery manager Bill Smith told the Lodi News-Sentinel. The utility’s own account of that season points to limited water available for pulse flows in a drier year.
A run of more than 730 steelhead shares the same managed water
While Chinook get the attention, the same system carries a quieter passenger. The hatchery reported a run of more than 730 steelhead in the same season the salmon count fell back, a result the utility credits to its management and release strategies.
That overlap matters, because California Central Valley steelhead are listed as threatened under the Endangered Species Act. The Mokelumne’s narrow plumbing is doing double duty, supporting a commercial fishery and a protected run through the same stretch of managed water.
A story about steelhead returning hundreds of miles upstream elsewhere in the West offers a comparison for how much one small dam can matter to two runs at once. Even in that smaller season, the hatchery still met its goal of collecting and fertilizing millions of salmon eggs with the state. Much like the camera based monitoring used to track predators elsewhere, the count here depends on one fixed chokepoint doing all the work.
A record run can fall back toward average within a single dry year
None of this guarantees the run stays strong. A dry winter upstream, a thinner pulse flow, or a shift in ocean conditions can pull a record season back toward average within a year, which is roughly what happened after the last record. The system is resilient, not invincible.
Even so, a river contributing a sliver of the delta’s water budget can still anchor a large share of the state’s commercial salmon catch. That is a small, strange kind of hope.
It suggests that careful, modest stewardship of even a minor waterway can punch above its size. So when the ladder at Woodbridge fills with fish again next October, the river doing the heavy lifting will not be the big one on the map.
Read the whole thing?
Get the week's signal, not the noise
Our sharpest reporting on energy, climate and nature — free, once a week.
