The phrase "upper basin" refers to the higher or upstream part of a river or water system. It is the area where the river begins or originates. Full definition
At that location, Colorado River streamflow reflects water that has drained from the upper basin, which includes Colorado, Wyoming, Utah and New Mexico. (sciencedaily.com)
But while National Oceanic and Atmospheric Administration research shows that snowmelt runoff into the upper basin hasn't been this high since 1986, the southern end of the Colorado River continues to stop shy of the Sea of Cortez, where it used to run until the late 1990s. (scientificamerican.com)
In addition to providing hydropower at Glen Canyon Dam, Lake Powell stores water from the upper basin states and releases it into downstream Lake Mead to ensure the upper basin states meet their obligations under the 1922 compact. (scientificamerican.com)