POST No. 2 - please read post no. 1 first
Based on the definition from the previous post, and the Missouri and Columbia Rivers once being used for interstate and foreign trading, I give you The Highest Navigable Lakes in the U.S. (at least that I could find)
LAKE OAHE, the 4th-largest man-made reservoir in the United States, extends from Pierre, South Dakota to Bismarck, North Dakota. The lake is 231 miles long, has 2250 miles of shoreline, and has a maximum depth of 205 feet. (Missouri River) Average elevation: 1608 feet.
LAKE SAKAKAWEA (Garrison Dam Reservoir) Missouri River, North Dakota elevation 1634 feet
FORT PECK LAKE, Montana (Missouri River) 2222 feet http://www.mrd.usace.army.mil/rcc/plots/pixel/ftpk.gif
CANYON FERRY LAKE, Montana (Missouri River) 3788 feet http://www.mrd.usace.army.mil/rcc/plots/pixel/cafe.gif
FRANKLIN D. ROOSEVELT LAKE -1284 feet -formed by the Grand Coulee Dam on the Columbia River, in Washington state. The Columbia had been used by explorers and traders for the Hudson Bay Co. and the Northwest Trading Co. Therefore, by definition, it is a navigable lake. http://www.nps.gov/laro/