Well, you have just touched on a subject that has a whole lot of "grey area."
The upper part of the river is in a National Wildlife Refuge. There is an federal prohibition on the possession and consumption of alcohol in a NWR, and that is strictly enforced by the USFW Agents there. Not much grey area there, pretty B&W.
Fort Niobrara National Wildlife Refuge: Nebraska
When the river flows through Smith Falls State Park, the Nebraska Game and Parks Commission has a regulation prohibiting the possession of open containers of alcohol. They only seem to enforce it when you step onto the land there, however. You can find that reg using this link (Select Game and Parks and enter "alcohol"):
Nebraska Secretary of State - John A. Gale It is also well-posted. Again, not much grey there.
The Middle Niobrara NRD does not allow alcohol at their landings. As a political subdivision of the state, the NRD must specifically authorize the consumption of alcohol on their property for it to be legal, something MNNRD apparently doesn't do (more on 53-186 later).
Most of the rest of the river there is a "National Scenic River" managed by the National Parks Service. While NPS prohibits alcohol in most National Parks, they seem to be mum on whether or not you can have it at "NIOB." This is all they have to say:
Alcohol -
Alcoholic beverages may be neither possessed nor consumed in the Fort Niobrara National Wildllife Refuge. Alcohol may be possessed, but not consumed in the Mid Niobrara Natural Resource District boat landing and in Smith Falls State Park. Private campgrounds may have thier own regulations. Remember that alcohol affects your coordination, balance and judgement, skills that are necessary to safely float the river.
http://www.nps.gov/niob/planyourvisit/rules-and-regulations.htm
Just to cloud the issue more, there is a state law, 53-186, that prohibits the consumption of alcohol on any property owned or controlled by the "state" or any political subdivision thereof. So, how much of that river is owner or controlled by "the state," besides what we have already discussed?
The private campgrounds, as private businesses, can choose to allow or prohibit alcohol in their campgrounds. If they sell it, they obviously have to be licensed by the state to do so.
As most people can attest to, enforcement is spotty at best all along the river. When there is enforcement they worry more about life jackets, MIP's, and obnoxious behavior than then average recreator with a cold beer. However, the Refuge is an exception. They can and do ticket for alcohol there quite often.
So just be willing to accept the consequences of your actions should you be caught and cited while drinking where you shouldn't be on the river!
The bottom line is also summed up nicely by the last sentence in the NPS paragraph:
Remember that alcohol affects your coordination, balance and judgement, skills that are necessary to safely float the river. Beyond the legal consequences, remember that "
the water is the true judge and jury."
Be safe!