Here, for example, is the reconstruction of the Colorado River's flow at Lees Ferry, Arizona, going back to 750 CE (scroll down for that graph).
Closer to home, the Arkansas River at Cañon City, Colorado, from 1685–1987.
The process correlates tree rings with observed data from the late 19th century to the present, then projects the correlation back over older tree ring samples from cores or archaeological sites. More about the process here.
The persistent drought conditions that emerged across the West in 1999, especially the extreme drought year of 2002, indicated that the observed records of streamflow in the region did not capture the full range of natural hydrologic variability. This drought, along with increasing demands on water supplies led to a need to assess the range of drought conditions that were likely to occur. Tree-ring reconstructions of streamflow, extending several hundred years or longer, provide a more complete representation of past variability. Accordingly, streamflow reconstructions attracted more interest within the water management community as a potentially useful tool for planning.