Head west from Turtle Bay Exploration Park toward the Sundial Bridge and the Diestelhorst Bridge. This is a nice paved path that follows the river. You can link into a number of other segments of the Sacramento River Trail system along the way.
Overall, this is a great paved trail that goes from Keswick Dam up to Shasta Dam. This trail follows Lake Keswick all the way up and it's fairly flat the entire way with the biggest climb found in the first 1/3 of the ride.