Cascade Lake Dam Information

Cascade Lake Dam is owned by Orcas Water Holdings LLC, which is affiliated with the resort’s former owner (per San Juan County parcel records, October 2026). See our Rosario Resort page for more on who owns what at Rosario today.

The information below is a subset of what is available from the Washington State Department of Ecology (direct link to Dam Safety page), the agency that regulates and monitors the construction and maintenance of dams in Washington State.

The following information was provided by a neighbor and is reproduced here without any warranty:

The Washington Department of Ecology manages a fairly strict dam safety program for most of the dams in our state, and both Cascade Lake and Mountain Lake dams are included. Since multiple people have mentioned concerns about dam safety, I pulled the latest information from Ecology to see what I could learn. Without boring you with all the details, my key take-aways are as follows:

Both dams are concrete arch dams, and, according to the U.S. Bureau of Reclamation, there is no record of any concrete arch dam ever suddenly and catastrophically failing…even after an earthquake. So that’s reassuring.

While the Mountain Lake Dam has been structurally assessed for its ability to withstand a certain amount of earthquake loading, the assessment was done before the current science around the Cascadia Subduction Zone quake. That quake could be twice as strong as what Mountain Lake Dam is rated to handle. A careful reevaluation has not yet been done, and it’s possible that the dam would hold up just fine. Beyond that, the Mountain Lake Dam is in pretty good shape considering its age.

Cascade Lake Dam has not been evaluated for its ability to withstand an earthquake, at least not recently. In any quake over 5.0 on the Richter scale, Rosario maintenance staff are supposed to do an immediate structural assessment and activate the downstream evacuation plan if the dam shows signs of imminent failure. The downstream danger zone includes the house on the south corner of Palisades and Cascade and a couple of condo buildings managed as guest rooms by the resort. Palisades Drive could get washed out. Seismic risk aside, the dam is in decent shape – though the resort hasn’t always been good about completing the required annual inspections and submitting the results to Ecology.

With both dams, based on past inspection reports, it seems to me that the biggest risk is that a spillway might get blocked by logs or debris during the rainy season without anyone noticing. During a flash-flood rain event, this blockage could theoretically result in a dam overtopping, which could wash out the dam from the downstream side. So keep your eyes open during the winter when walking by our dams, and if you see something – say something! If the lake water level is ever more than 6 inches above the lower lip of the rectangular spillway, or if the spillway is actively eroding, those are grounds for immediate concern.

Cascade Creek, which flows out of the Mountain Lake Dam, does not flow into Cascade Lake, except for a diversion managed for the benefit of Rosario Resort’s power plant; the creek actually flows south, out into Buck Bay. Theoretically, a failure at Mountain Lake Dam wouldn’t impact the Rosario area at all, except that the recharge canal would stop flowing and the resort, in the summer, would have to shut off its hydro-electric plant to avoid (relatively) quickly drawing down Cascade Lake.