In September, when most of the rivers in Washington have slowed to a trickle, river rats from around the Pacific Northwest gather around the normally quiet Tieton River for the big yearly event they call the “flip flop.” That’s when the engineers over at Rimrock Dam push a button and unleash a torrent of water that drains through the Tieton out to the greater Yakima River drainage area to provide Yakima Valley farmers valuable irrigation supplies.

Rafters reap the benefits with an exciting class III (intermediate) ride that starts high in the mountains and descends at a rapid 50-feet-per-mile pace down through basalt canyons and into the dry valley on the east side of the Cascades, where the Tieton meets Naches River. The flip-flop only lasts about a month, but some rafting companies will still take you out through October.

There are dozens of rafting companies with encampments up and down the river. Get the most bang for your buck from Blue Sky Outfitters (800/228-7238, www.blueskyoutfitters.com), which paddles an epic 21-mile run along the river—longest among all guide services out there. Blue Sky also serves up a generous gourmet barbecue dinner after all is said and done aboard the rafts.

Other respected services include Downstream River Runners (800/234-4644, www.riverpeople.com, $75), which employs some of the most experienced guides on the Tieton and provides snacks before and after your trip, and Alpine Adventures (800/723-8386, www.alpineadventures.com, $84), which has arranged special weekend packages with Yakima innkeepers.