The Colorado River is often referred to as Arizona's West Coast as the river originally served as the border between Arizona and the neighboring states of California & Nevada. Examination of Yuma area maps finds several discrepancies in this rule. This is due to the meandering of the river, the channelization of the river, and the lowering of river levels due to upstream diversion dams.
In the early 60's California and Arizona agreed on a new definition of the common border that did not strictly follow the path of the river.
As a result of the new border definition the Paradise Casino was originally able to be built so that it lied half in California and half in Arizona! Recently, the California side of the Casino was closed so that the new Q casino near Andrade could be operated.
There is an 8 mile section of the Colorado River above the Ocean to Ocean Bridge that is found entirely in Arizona. Further upstream from just below the Laguna Dam all the way to the Imperial Dam the river channel is found in California.