Just kidding; I’m not actually going to spend 1,000 words on a river surf wax review. While I’m usually happy to overthink any plate of beans when it comes to river surfing, for the most part surf wax is just surf wax. Some like fixed rubber or foam traction pads on their boards, some like wax, some like neither — I usually use a mixture of both (a traction pad on the tail and wax for my front foot). I’ll usually wax foam boards as well, since I like the little bit of extra tackiness. Your mileage may vary.
Pay attention to water temperatures, recorded on many (though not all) of the USGS gauges. For example, the best flow gauge for River Run Park doesn’t track temperature, but the Englewood gauge a few miles downstream does:
Most surf wax will have a recommended temperature range. In the summer I’ll lean towards slightly warmer temperature wax than the water temperature might otherwise indicate, since the Colorado sunshine will try to turn cold water wax to goo. Usually that means cool water wax early in the season and warm later in the season.
A wax comb can also come in handy if you want to get some texture back from old wax, but a cheap drug store comb will work in a pinch.
Some people have strong wax brand allegiances. There are non-toxic, biodegradable waxes out there like Matunas. Sex Wax and Sticky Bumps are probably the two most popular brands. But basically go with the wax in the right temperature range that smells the best or is cheapest or both.
A couple of companies have also come out with wax specific to foam boards, though I haven’t heard anyone claim these formulations have any particular magic to them.