Scientist tell us that we can determine the state of a waterway by its color. Color is closely linked to water quality, indicating the amount of dissolved carbon, pollution, sediment and algae. The bad news is that 56% of the rivers observed by satelite (67,000 miles) have turned from a healthy blue color to green or yellow according to a study published in Geophysical Research Letters. Only 6% were found to be blue. 55% of the rivers did not exhibit an long term trend in color change. Steady color change was found mostly in the central US and lower Mississippi River Basin. [see chart].
Scientists are not sure about explanations for the color shifts. They point out that rivers and lakes connected by water projects become sediment traps and promote the growth of algae which in turn reduces oxygen levels in the water bodies. Among rivers within the top 25 percentile of rates (for example, faster rates of change), 45% are near a dam (< 25 km upstream or downstream) and 22% are located within urban areas. Among rivers within the bottom 25 percentile of rates, only 12% are near a dam and 7% within urban areas. They also suggest that climate changes plays a role as rivers flow through regions of increased aridity. They elaborate on their findings, “Fragmentation and transitions in color along rivers are important because the color is likely linked to ecological function. The observed spatial patterns in color may be indicative of primary productivity regimes, suspended sediment, as well as light availability for photo-degradation.”