(1) Bitterroot River is closed to use for any motor-propelled watercraft in the following areas:

(a) Missoula County from the Ravalli County line to its confluence with the Clark Fork River with the following exceptions:

(i) any motorized watercraft may be used from May 1 through June 30 on the portion of the Bitterroot River from the Florence Bridge in Ravalli County downstream to the Clark Fork River;

(b) in Ravalli County from the Bitterroot River headwaters to the Missoula County line with the following exceptions:

(i) any motorized watercraft may be used from May 1 through June 30 on the portion of the Bitterroot River from the Florence Bridge in Ravalli County downstream to the Clark Fork River;

(2) Motorized watercraft powered by 15 horsepower or less may operate anywhere on the Bitterroot River from October 1 through January 31.