Is the sandbar permanent? If so, it would be treated like an island. Are there similar islands in the Platte? If so who owns them?
If the sandbar is temporary, is it surrounded by private property? I think that the answer is going to depend on who you are likely to bother by putting up your tent.
Nobody around? No problem. In the middle of a busy public park? Maybe a problem.
Generally speaking, most states (I think?) allow right of passage along navigable waterways, even if that waterway goes through private property. This means that if you are canoeing, for example, you have the right to navigate your canoe along the Platte River. If there’s a sandbar blocking your path, you have the right to portage your canoe around or over the sandbar. The same goes for other obstacles. If there is something blocking your canoe and you need to drag it up onto the shore onto private property to get around the obstacle, that is not considered to be trespassing.
This is generally only a right of passage though. If you stop on the shore to rest and that shore is on private property, that’s trespassing. Similarly, if you stop and camp on a sandbar and that part of the river is owned by someone, that’s trespassing. Even if you drop anchor and stay in your canoe, that’s trespassing (you’re stopping, not navigating). If you are dragging your canoe around an obstacle, you only have the right to do what is necessary to navigate the waterway. So you can drag your canoe over a section of shore to get around an obstacle, but you can’t stop and camp there because of the obstacle as camping isn’t necessary for travel and that would be trespassing. Also, if the sandbar isn’t blocking your path, there’s no justification for trespassing on it in order to navigate the waterway, so that would also be trespassing.
The state’s department of natural resources should be able to tell you what the rules are with state-owned sandbars on the Platte River.
That takes us back to the old problem of asking and being turned down. Most of us discovered at some time in our youths that If you did something wrong, where there was no clear rule against it, you usually got away with it. Breaking a law or rule that you clearly knew about was much worse.
I suggest that the OP camps out on the sandbar and if some official tells him to move on - just pack up and go. I doubt that anyone would actually know if he is there for one night. Discretion would be sensible though - taking a bunch of mates, building a bonfire and having a party may be asking for trouble.
Maybe that depends on the season. What I read says that the state owns the river and banks up to the high-water mark (low water mark?). They hold them in trust for the
Generally, AFAIK, IANAL, the ownership extends to the river bank and the government “owns” the waterway. …which does not answer the question of what happens as the river bank moves or a sand bar pops up.
There’s a number of places, particularly the Mississippi, where a small chunk of one state is across the river from where it should be. But that is not so much the river moving, as re-routing so a high and dry piece of land is now on the other side.
On such trivialities are fine points of law and lawyer incomes made.
If you were asking in general, this would be a really complicated question. River law in the United States alone derives from at least three European codes and has been specialized for each state, with exceptions for all sorts of rivers and unique situations. Since you asked about the Platte (and not the North or South Platte), I assume that you’re asking about Nebraska law.
Usually, there isn’t any problem with camping on a sandbar in a US river, but every few years you hear of someone getting shot by an owner defending their actual or perceived rights. Personally, I’d call the owner of the property adjacent to the river and ask.
A friend who guided overnight trips on Western rivers swears by sticking a battery operated water alarm, like what you put in your basement or beside the water heater, between the river and your stuff.
Thinking about it, much of the Great Plains is composed of silty dirt. That sandbar may be a pretty nasty mudbar instead.
In the law of nations, the standard definition for a river border is that it follows the Thalweg (valley way), which is the line connecting the deepest points along the channel. So even if the river dries up, you can still find the border. That just occurred to me.
Other places may distribute the river differently. Thanks to the charter given by King Charles to Lord Calvert, Maryland and DC have the whole Potomac, while the other side, Virginia, doesn’t get any and doesn’t start until the Virginia shoreline.