For Boat OdysseyGREAT FALLS. Mont.Two former Air Force men hitchhiked into Great Falls: Tuesday, parked their kayaks: and took a tourist's detour to:Yellowstone National Park be-:fore resuming a transcontinental river trip.Jim Cumes, 23. of Wheeling. W.Va., and John Davidson, 21. of Carlisle, Pa., planned to head: down the Missouri River early next week.They paddled 550 miles up the: Columbia. Snake and Clearwater rivers to Kamteh. Idaho. From there, they hiked 600 miles to Missoula and caught a ride to Great Falls.After a tough upstream puU onj the Clearwater, they abandoned; plans to travel the Clark Fork: River from Missoula to Garrison. They had wanted to portage over the Continental Divide and. reach the Missouri River near Helena.Cumes and Davidson hope to reach Chesapeake Bay by latc| October after paddling the Mississippi. Ohio, Monongabela andj Potomac rivers. They started! May 18, I