Streamflow data are essential for water resources planning and decision making and are routinely analyzed to determine the impacts of climate change on hydrology. Unfortunately, current stream gauges, largely the responsibility of the U.S. Geological Survey (USGS) in the U.S. and similar agencies worldwide, are expensive to install and operate and are being steadily decommissioned. Part of the solution to this problem is a low-cost stream gauging system that is simple enough to use by people

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... h little or no formal training in environmental monitoring. In this article, a low-cost, digital camera-based stream stage monitoring system is proposed, described, and tested. As a proof-of-concept, a time series was generated by taking digital pictures of a staff gauge at 3 h intervals over several weeks at a current USGS gauging site. The image-based stage heights closely matched the USGS gauge values, although significant stage height errors were evident in a small percentage (<3%) of the images. We identified the problem as being caused by shadows and irregular lighting and proposed a protocol for eliminating these errant images. When the obviously problematic images were removed, the relative differences between the image-based stages and USGS stages were approximately 5%. The next step is to develop an on-line system for post-processing the images so that watershed networks, citizen science organizations, K-12 educational institutions, and others can engage in stream monitoring and make their data freely available. We also propose some possible next steps for determining stream cross-section and flow velocity using this low-cost camera-or image-based monitoring system.