The Bonneville Power Administration (BPA), the United States Army Corps of Engineers (USACE), and the U.S. Bureau of Reclamation (USBR) perform hydroregulation studies of the Columbia River basin for analysis of environmental impacts, changes to operation criteria from Biological Opinions, power revenue forecasts, flood control studies, operations planning, downstream benefit calculations, and effects of new projects or plant data. Also, a wide range of other regional organizations, including the Northwest Power and Conservation Council, Northwest Power Pool, Pacific Northwest Utilities Conference Committee, fishery agencies and organizations, universities, research organizations, contractors, and public interest groups have a need for a consistent and accepted regional streamflow dataset.
Modified flows are defined as the historical streamflows that would have been observed if current irrigation depletions (as of year 2008) existed in the past and if the effects of river regulation were removed (except at the upper Snake, Deschutes, and Yakima basins where current upstream reservoir regulation practices are included). However, irrigation practices have changed since the historical flows were observed and so the historical streamflows have been adjusted to account for current levels of irrigation depletions. The 2010 modified flow study includes 80 years of flows (1928-2008) adjusted to 2010 irrigation depletions.