two ideas

Based on the nature of the river & the water levels, the water itself gets classified as Class 2, 3, 4, etc. Experienced kayakers know all this (maybe it's documented somewhere?? Not sure. Whoever is managing the app would enter this info - usually it's a handful of local sites that clubs frequent). Say a section of a river is generally classified as Class 3, if the water level happens to be exceptionally high during a period of time, then it may be considered Class 4 & people may restrict the trip to those who have experience or are ready for Class 4 challenge. Folks who kayak know this info, but less experienced ones may not know to make these judgements. So my idea was that an experienced kayaker from a club (for ex, a whitewater safety chair from a club with which I used to kayak) could log in during certain intervals & note that a particular river section that's normally Class 2 this weekend is running at a Class 3 level. So if less experienced people were planning a trip to such a place, they would check the app (maybe receive a notification if they've documented a trip or just signed up for alerts during a particular time for a particular river), have a heads up & possibly consider other locations, restrict the trip to certain participants, or just cancel. Or if the water is too low & people would just end up scraping their boats on the river bottom, they'd want to cancel the trip for no fun.

The university club where I used to kayak was really looked up to within the local community & surrounding areas within about a 5 hour drive for their expertise & safety protocols. So if this type of documented info was available to others to access, I think it would prevent water related accidents among the general community of kayakers who go off on their own without experienced kayakers & really understanding the risks.