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Importance of Long-Term Strearngages Protection of environmental resources has become Long-term streamflow information is critical for several a major objective for water managers,including providing ade- types of studies that benefit people and wildlife. These g quate flows for fish and other aquatic organisms(Hester and uses are described in more detail in the following"Water others,2006a).Long-term streamflow information is needed Management,„«Design Flood Flows, and Climate-Related to understand the variability of streamflows,particularly low Trends"sections. streamflows,and how managed flows may differ from natural flows. Water Management Design Flood Flows Consistent,long-term streamflow data are important for Designers of bridges, culverts,dams,flood lain struc- allocating water according to agreements among water rights g g p holders,including interstate and international agreements tures, and other important infrastructure need to compute the g g size of flood flows(such as the 1-percent annual exceedance (Hester and others,2006a).As an example,30 million people probability flood,which is also known as the 100- ear flood) in the and southwestern United States depend on Colorado p y y agriculture,Indust that will affect those structures.Long-term streamflow records River water for home use industry, and recreation usingconsistent measurement techniques provide a high (Jacobs,2011).The history of streamflow and water rights on q p g the Colorado River shows how long-term data are needed to degree of confidence in the calculation of these design flood g flows(Hester and others,2006a),minimizing the safety risks understand long-term streamflow variability. It is now known of under design and the costs of over design. that the 1922 Colorado River Compact,which governs water g g allocations between the u er and lower Colorado watersheds, Reservoirs across the United States are important for pp water supply,flood protection,hydropower, and other uses was based on a short period of relatively high streamflows Hester and others,2006a). Streamflow data are needed to National Research Council,2007).Figure 1 shows the low ( � ( ) g size reservoirs; long-term data are needed to deter- water level in Lake Powell on the Colorado River in 2004 properly g that was caused b five consecutive ears of below average mine the size and variability of flood flows into the reservoir, y y g as well as the amount of water during very dry or wet periods. inflows(Fulp,2005). 120,000 100-year peak flow, based on 1947 to 2012 data 100,000 100-year peak flow, based on 1947 to 1971 data • 0 0 y 80,000 • • CD • • • U • • 60,000 • • • • • • • CU • 40,000 • • 20,000 0 1950 1970 1990 2010 Year Figure 2. Annual peak flows for St.John River at Dickey,Maine,and 100-year peak flows based on data from selected periods.