This is probably going to be a stupid question, but is there a web resource wheren somebody can find the average current speed of certain rivers? Or some kind of paper resource?
The speed of the current is going to be highly variable, both along the river (slow where the river is broad and deep, fast where it is narrow and shallow) and across it (high in the center of the main channel, low near the inner banks, etc.). Measurements of flow volumes (usually in cfs, cubic feet per second) are made at “gaging stations” along many rivers; Google for your region or specific river (like this) and you might find something. If you can estimate the cross-sectional area of the main channel you can divide by that to estimate the speed.
If it’s a river that people want to paddle, then you can bet there is a ton of info on the web about it, like its volume (cubic feet per second, or cfs) or “level,” which is the mark it is hitting on some physical gauge. I don’t know if there is as much information about actual current speed out there, but who knows.
Try americanwhitewater.org. Go to “river tools.” Click on the state where your river is, and you can get info on the river’s flow.
The USGS site also has a lot of information about particular rivers.
If nothing else, you might be able to find the right person to ask about the actual current speed. If there are outfitters running commercial trips on the river, try them before you try governmental agencies. Outfitters tend to know a LOT of trivia about their rivers because customers ask all sorts of questions about the rivers.
Ompha is correct, of course, about the current speed being variable depending on the part of the river you’re measuring. But if you want to know the average current speed for a given section at a particular volume, someone might well be able to tell you. Back when I was a guide, I knew the average current speed for the section I was working at the most common levels.
Try the USGS (US Geological Survey) site. You won’t get the current flow as mentioned in the previous posts but you will get a bunch of other data.
If you want to measure current you can get one of these..
In the US, the National Weather Service also has a lot of river height & flow volume info on the net. Here’s a random sample from a small town on the Mississippi near me: http://www.crh.noaa.gov/cgi-bin/ahps.cgi?lsx&cagm7 You can root around in there and probably find links to similar info for anywhere else in the country.