Below La Grange , the Tuolumne – much reduced due to irrigation diversions – flows through a fixed channel with a highly restricted floodplain . Before agriculture , the river inundated large areas of seasonal marshland and was free to change course during its frequent floods . Today farms and communities are protected by a levee system with a channel capacity of roughly at the 9th St. Bridge in Modesto – significantly smaller than the river 's maximum flood potential . The valley is dependent on flood control storage at Lake Don Pedro to prevent flows from exceeding the channel capacity . However , this has happened many times , most infamously during the New Year 's Flood of 1997 , during which inflows to Don Pedro hit , the largest recorded since the Great Flood of 1862 . Water releases from Don Pedro Dam peaked at ; while this vastly reduced potential damage from the flood , it still exceeded the channel capacity by six times , causing extensive property damage along the river
Great Flood of 1862
Great Flood of 1862
Tuolumne River