Flooding on the Nisqually River is related largely to the amount of water released from LaGrande Dam (in Pierce County near the southeast corner of Thurston County). This, in turn, is related to how much water enters Alder Lake and is released from Alder Dam. Feeder streams such as Ohop, Yelm, and Taxwax creeks also influence flooding, as does high tide in the Nisqually Delta.
The National Weather Service issues a flood warning for the Nisqually River when their forecast model indicates the river will reach a stage of 12 feet or higher at the McKenna gauge. Minor flooding (low-lying roads and pasturelands) occurs when the flow rate at La Grande exceeds about 8,000 cfs (cubic feet per second) or a gauge height of about 8.75 feet at the McKenna gauge. Moderate flooding (individual residences are threatened) in Thurston County occurs when the flow rate exceeds about 15,000 cfs or a reading of 10 feet or over at the McKenna gauge. A high tide could flood the banks at Hayko Lane, 6th Avenue, Riverbend Campground and Riverside Manor apartments. Major flooding (widespread threat to communities and major thoroughfares) occurs when the flow rate tops 16,500 cfs or a gauge height of 10.5 feet at McKenna.
Recently work was done in the Nisqually Delta to restore the natural estuary habitat. It is unknown exactly how this reclamation will affect the anticipated flooding impact levels shown below.