From author & historian Hutchings 1862: "...we approach the new and flourishing little settlement of Rio Vista. “This town,” writes Dr. C. A. Kirkpatrick, the obliging postmaster, “is situated about forty-five miles below the city of Sacramento, and below the outlets of all the large sloughs, or at least two of the largest, Steamboat and Cache Creek sloughs—uniting with the main, or old Sacramento river, just above this place; making the stream here about one-third of a mile wide." SCENE AT THE LOWER JUNCTION OF THE MAIN SACRAMENTO RIVER, AND STEAMBOAT SLOUGH.

Steamboat Slough was originally referred to as the "Middle Fork" of the Sacramento River, (click below) according to the map from 1852, and official surveys for the federal government by Mr. Ringgold, chief surveyor at that time. However, very soon thereafter, maps began referring to the waterway as Steamboat Slough.   A noted author of the time, Mr. Hutchings, in his 1862 book of California refers to it as Steamboat Slough in his sketches..  http://www.yosemite.ca.us/library/scenes_of_wonder_and_curiosity/ alabaster_cave.html The 1949 Guide book of "California Place Names" says on page 320:  "Steamboat Slough {Yolo}  When the Sacramento was first navigated fewer obstructions to navigation were encountered in Steamboat Slough than in old Sacramento River, as the other branch is called.  For many years the slough was therefore the channel preferred by navigators...."    You can read history of the Delta King and Delta Queen steamboats for a sample of river travel back then.

Historic Shipwrecks of Steamboat Slough