Go to www.texaswatersafari.org/course-map/
Below are local forcasts for several cities along the path of the San Marcos and Guadalupe rivers. Click on each for an extended forcast. Also, Weather Underground provides a weather map service that ties together, among (many) other things, radar, temerature, wind, and river flow.
Below are all the flow gauges on the San Marcos and Guadalupe rivers that are part of the Texas Water Safari course.
For more information check out http://www.texaswatersafari.org/