|How to use CNA (Comision Nacional del Agua)
There is no online river flow information for Mexico. The rivers are primarily fed by monsoon thunderstorms, so their flows go up and down all the time during the rainy season (Jun-Oct). The government does have current river gauging and flow information, but it is not easily accessible to the public and not online. I'll be trying to change this, but for now you must rely on weather reports and forecasts to figure out which rivers have water. CNA is the national agency that deals with the water in the republic and provides information about rainfall, climate, etc.
Unfortunately, I cannot provide direct links to the most relevant pages on the CNA site since they have them within frames without their own URL. I will describe how to navigate their site for relevant information.
First, the most important place to find relevant information after opening the main CNA page is to click on the "productos" link at the top. This will open up another page with a lot of links. Within this page if you click on "mapa diario de precipitation" you can get to a page showing in general where rainfall fell for the previous day. They also have it for week (semanal) and month (mensual).
Second, probably one of the nicest links is to the experimental product called "Hydroestimator". This takes Doppler data and other data to make a map of how much rain fell and where in the past 24 hrs. After you open this link, you should click on ESTIMACIÓN DIARIA ACTUALIZADA, which will show the previous day rainfall. This is extremely useful for predicting which rivers will have water and flash floods, essential for monsoon-fed rivers.
Third, on the main CNA page you can click on "Boletines" which gives forecasts (through the Boletín Meteorológico General) and rainfall amounts for various locations, including the highest amounts in various parts of the republic (through the "Informe Meteorológico de Lluvias" link).
Remember rules of thumb for conversion. 25.4 mm = 1 inch. 30 cms = 1059 cfs.