At some flows, you can actually make it work, but it is a little tricky, requires a bit of a hike, and may be detrimental to your health. You take Riverside Parkway to the light associated with the short stretch of 4th Ave. that dead ends at the railroad tracks. At the dead end, drive on the dirt "road" next to the tracks going under Riverside Parkway. You can park on the side of Riverside Parkway closest to the river. Unfortunately, this is basically in the middle of a prairie dog town, so try to avoid catching bubonic plague. From there, you can hike your boat to the river. Unfortunately, you will be at a small side channel and will need to ferry across to the island. The problem is that the head of the island, where you will want to go, may be flooded with water. Depending on the level, though, you may be able to hike across the island to the main flow, where the 5th St. wave is on the other side of the river. If the island is too flooded with water, you may still be able to use this parking spot, provided that you hike a little further upstream to the main flow.
P.S. I have stopped using this parking spot due to my fear of bubonic plague, and I get a ride or use a bike to shuttle back to the botanical gardens. The bike ride is very short.