The lower is still accessible, but only at certain hours. i think until 7 on weekends and 4 on weekdays. They will impound any vehicles left inside the gate after they shut you out, but you can still park at the top of the hill and walk back in, without any problems, as long as you are on the river by "closing time".
"Believe me, my young friend, (said the water rat somlemnly), there is nothing - absolutely nothing - half so much worth doing as simply messing about in boats. Simply messing...." -The Wind in the Willows