Stand beside almost any mountain creek in Colorado, and there’s a good chance no one has ever measured how much water is flowing past your boots. Of the thousands of small headwater streams that lace the state’s high country, feeding its rivers, irrigating its farms, and sustaining fish and wildlife, only a small fraction has a streamflow gauge. For the rest, knowing whether a stream is running high, low, or somewhere in between has traditionally meant a trip into the field and an educated guess.  Researchers in the Department of Ecosystem Science and Sustainability, ESS, have built a new tool designed to close that gap. CSUFlow25, a free, publicly available web application, lets anyone drop a pin on a Colorado stream and get a statistically modeled estimate of how that stream flows across the year, no gauge or field visit required.