Some critics charge that the dam has strongly impacted the runs of salmon and other migratory fish in the Sultan River by depleting gravel and sediment needed to line the riverbed.

This supply proved insufficient as well, so engineers looked to tap the main stem of the Sultan River, where an excellent dam site had been identified.

The construction done during this phase served to replace the smaller, 22-foot (6.7 m) diversion dam 6.5 miles (10.5 km) downstream[4] on the Sultan River.

[8] In June 2016, the PUD began demolition of the dam's sluiceway to re-open the upper Sultan River basin to spawning fish.

[9] Culmback Dam and Spada Lake received water from a drainage basin of 84 square miles (220 km2) on the west slope of the Cascade Range.

Additional major contributors are the South Fork, which forms an arm of the lake close to Stickney Ridge, and Williamson Creek, which enters from the north at the base of Big Four Mountain.

[12] The area around the dam and Spada Lake is visited by 5,000 people annually for a variety of recreational activities, including boating, hiking, fishing, and kayaking.