Research Aids River Fish
Dec 23, 2025 11:40AM ● By Susan Maxwell Skinner, photos by Susan Maxwell Skinner
SACRAMENTO REGION, CA (MPG) - A year-round survey is keeping tabs on oxygen levels in the American River.
Operating with Sacramento Water Forum funding since 2022, the project measures dissolved oxygen from below the Folsom Dam to Nimbus Basin; surveys also extend to Fair Oaks and Watt Avenue reaches.
Although dissolved oxygen levels are critical to all river health, research is particularly supportive to Chinook salmon reproduction.
Teams of biologists engage in daily readings. This reporter accompanied them on a recent river outing.
A critical salmonid spawning habitat lies below Nimbus Dam. Here, shallow gravel beds challenge data-gathering.
Biologist Mitch Gladding uses an oxygen-measuring device in Nimbus Basin.
Carefully avoiding teeming Chinook, Cramer Fish Sciences staffers drag an inflatable craft through channel riffles.
“We watch where we’re walking” said Biologist Mollie Ogaz. “The salmon redds (nests) are easy to see -- if you know what to look for.”
Oxygen sensors trail in the biologists’ wake. Ideally, dissolved oxygen should measure more than 7 milligrams per liter – or about 70 per cent of water content.
“If oxygen falls below that level, salmon can become stressed,” said Water Forum Program Manager Erica Bishop. “We don’t want these Chinook adults to die before they spawn. Egg and fry development also suffers if water conditions are not sustained.”
Year-long monitoring influences Nimbus Dam spillage; data also assists Nimbus Hatchery operators.
“The Cramer team reports impact decisions affecting the fish,” said Bishop.
As mighty as migrating Chinook seem, the giants reach Nimbus Basin in a degraded state. They have not eaten since leaving the Pacific Ocean. Every ounce of remaining strength must battle currents, carve gravel nests and achieve spawning. With flesh breaking down, reproduction is a race against time. Low water oxygen can exhaust – even kill – Chinook before their natural life cycle ends.
“Their reproduction is a process that’s been happening for millennia,” explained Bishop. “We can help sustain it by gathering critical data. Among Water Forum objectives is protecting river health. Every species benefits from a healthy river.”
Trailing oxygen-measuring devices, scientist Mollie Ogaz and research partner Mitch Gladding navigate a riverside channel.
Various conditions contribute to low river oxygen. Fertilizer run-off creates algal blooms that gobbles the life-sustaining gas. Cloudy weather stalls plant photosynthesis and oxygen production. After storms, vegetation washes to rivers; its decay eats more oxygen. On windless days, oxygen diffusion is slowed. From giant fish to microscopic organisms, all river life suffers when oxygen content in the artery diminishes.
As in an aquarium, an influx of bubbles is a tonic. When Nimbus Dam gates open, crashing water quickly aerates fish habitats.
“We’re fortunate that (Nimbus Dam operator) the Bureau of Reclamation is a cooperative partner,” said Erica Bishop. “We can all unite around solid data. The fish don’t know it, but good science leads to good outcomes.”
Oxygen level surveys are funded by the Sacramento Water Forum. Learn more at www.waterforum.org
