Yesterday I took some time on my way in to stop by the construction site on the river path (where Rock Blvd. crosses the Truckee). The guy I spoke with didn't know the completion date, but was able to tell me that they are installing a concrete upgrade to the path in this location and some low walls along the outside edge of the path. He didn't know the purpose of the walls (neither do I). I'm guessing it's a flood control thing??
My only real gripe is that they let you ride most of the way through the park before they tell you the path is closed. This leaves any trail user to turn back and retrace their steps to get out to the park exit. The worst part about the "detour" associated with this closure is that you end up on the wrong side of the river with no way to get back to the trail! It's not a huge deal, you can use the bike lane on Mill St. and then cut back North on McCarren to get back to the path, but not everyone is going to know about this unposted and unmarked "detour". So if you need it there's your alternate route.