This sub-region begins just west of Sacramento, extending into the California Coast Range, and encompasses roughly 2500 square miles. All the watersheds now end in the Yolo Bypass, an overflow channel for the Sacramento River. Historically all three watersheds drained into the Sacramento River in winter, so that forms its eastern boundary. The upper watersheds, in the Coast Range, are clearly divided from other watersheds by the mountain ridges. However, the watershed boundaries blur in the Central Valley.
Relief is very low in the valley, so there are no clear divides.
Also, the creeks and sloughs overflow during regular winter floods, and recharge the groundwater during summer. Thus the watershed area is not necessarily a question of what drains into the creeks, but what flows out of the creeks. The entire watershed has a Mediterranean climate, temperate and rainy in winter, hot and dry in summer. The upper part of the watershed includes mountains high enough to get snow, while the lower, Central Valley part, has night frosts only a few times a year. Summer highs regularly rise above 110 F. Because the Central Valley is narrow, inversion layers are common, trapping fog in winter and smog in summer.