Total Runoff Is Increasing but Shows Substantial Regional Variability

Average flows of streams and rivers in the United States have increased in most areas over the past several decades, which is consistent with observed and expected trends in precipitation. There are regional differences, however, with decreased stream flow in the Columbia and Colorado Rivers, for example. Observed changes in stream flow reflect both natural variability in hydrology as well as the aggregate effects of many human influences, of which climate change is only one. In some areas, changes in climate are exacerbating decreases in river and stream flows that are already declining due to agricultural, residential, and other human uses.

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