For the record, horsepower reduction for altitude is 3% per 1,000 feet of altitude above sea level. Significant power loss is not readily evident until you get past the 3,000 foot elevation. The elevation of the Snake River at Walter's Ferry is 2,264 feet so your 35 HP engine will net about 32.6 HP. Jetting the carburetor for altitude is generally not needed (or available) below the 5,000 ft range (this depends on the engine manufacturer).
Do you know why Cajuns don't hunt elephants? Because the decoys won't fit in their pirogues!