They made a winter crossing of the Carson Range and Sierra Nevadas in February 1843. From Carson pass they followed the northern Sierra's southern slopes, to minimize snow depth, of what is now called the American River valley down to Sutter's Fort located near what is now Sacramento, California. Fremont took the data gathered by his topographers and map makers in his 1843-44 and 1846-47 explorations of much of the American west to create and publish (by order of Congress) the first decent map of California and Oregon in 1848.