Its about a 15 minute boat ride from Topock to the Needles bridge at about 45-50mph...about another 10-12 minutes up to the AVI...you'll want to stay to the arizona side, the Cali side seems to have more submerged trees and junk...there are a couple of sandbars but if the river is up they are no problem...if its kinda of low watch for the big ripples, if it is real low then I wouldn't go up there with a prop...
Also, if you go in summer you may find no place to beach your boat at the AVI...you can always tie off to the dock and go get a pizza..then float back to Needles with pizza and beer!!