Long-term water color and flow trends in the Mississippi River Headwaters, 1944-2010

Intensification of brown color in surface waters has been observed over several decades in many areas. We examined a 64-yr daily record (1947-2010) of visual water color, a measure of chromophoric dissolved organic matter (CDOM), in the Mississippi River at Minneapolis, Minnesota. Although no monotonic trends in daily or mean annual color were evident, our analyses revealed trends in seasonal metrics, for example, mean winter color, on decadal scales related to changes in flow (hence climatic conditions). A pattern of high color (CDOM) in late spring and summer, corresponding with higher flow, was found across the period. Daily flow accounted for similar to 50% of the variance in color, and a lag of four days was found between peak responses of flow and color, supporting a CDOM source from wetlands in northern parts of the basin. The slope of the color-flow relationship increased over the 64 yr, driven by increased CDOM flushing in late summer-early fall. Based on trends in seasonally aggregated color and discharge, minimum and mean color and flow increased during winter over the 64 yr, potentially due to higher temperatures. Summer months did not show increases, but color became less variable. As a result, the color difference between summer and winter became smaller over the study period. During high flow events (ice-out or high precipitation), some hysteretic color patterns were observed consistent with observations on other large rivers. Our results indicate that long-term color (CDOM) trends in the Mississippi Headwaters reach are related to seasonally dominant changes in climatic conditions.