“Our ignorance of the route was complete. We knew that California lay west, and that was the extent of our knowledge. Some of the maps consulted, supposed of course to be correct, showed a lake in the vicinity of where Salt Lake now is; it was represented as a long lake, three or four hundred miles in extent, narrow and with two outlets, both running into the Pacific Ocean, either apparently larger than the Mississippi River.” (Echoes of the Past, p. 111)
The map shown above (which can be examined in its entirety at the University of Tulsa website) doesn’t correspond exactly to Bidwell’s description, but it does show two large rivers flowing from the Rocky Mountains unimpeded to the Pacific Ocean. One is labeled R. Timpanogos, and the other R. S. Buenaventura. Other maps of the period are similar.
The legend of these non-existent rivers had a fairly long life in American cartography. Explorers and settlers wanted a river that would link the Rockies to the West Coast, and make trade and transportation possible. They fastened on reports of much smaller rivers in Utah, like Green River, to create these waterways to the Pacific. You can see from the map that the distance from the Rockies to the Pacific is underestimated, and the Sierra Nevada range is not shown at all. The distance north to south, from the Columbia River to San Francisco Bay (labeled Port Sir Francis Drake) is also much less than it actually is.
With what feelings of dismay and alarm did Bidwell and his companions face the Nevada desert and the looming Sierras? By this point they knew the maps were wrong, and they were alone. Entirely dependent on their own resourcefulness, they traveled onward, with no way of knowing how far they had yet to go.