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36 <br />changing definitions of sustainability affect our understanding of water supply availability. Chapter 7 <br />includes a process to continue this type of evaluation in partnership with communities and other <br />stakeholders. <br />Surface water <br />The region's most visible water supply source is its surface water. Three major rivers, hundreds of <br />streams and ditches, and thousands of lakes and wetlands provide varying amounts of water. This <br />Master Water Supply Plan focuses primarily on one surface water source, the Mississippi River, but <br />also provides information about two other large potential sources: the Minnesota River and the Saint <br />Croix River (Figure 16). <br />Figure 16. Metro area's three major rivers, sized relative to the amount of flow that is likely to occur at least 90% of the time. <br />Use of the Minnesota, Mississippi and Saint Croix rivers is limited by a variety of climatic, economic, <br />water quality, regulatory, and ecological reasons. <br />For example, while average annual flow for the Minnesota (at Jordan, MN), Mississippi (at Anoka, MN) <br />and St. Croix (Stillwater, MN) rivers are 4,200; 9,000; and 3,100 MGD respectively, flow varies <br />considerably through time. 90% of the time, flow in the Minnesota River at Jordan exceeds 320 MGD; <br />flow in the Mississippi River at Anoka exceeds 1,940 MGD; and flow in the St. Croix River at St. Croix <br />Falls exceeds 1,290 MGD (Dadaser-Celik and Stefan, 2009). Another way to consider low flow is the <br />"7Q10" value. This value refers to the lowest consecutive seven-day flow that a river experiences on <br />WATER SUPPLY MASTER <br />PLAN- Draft June 2015 <br />