Know Your Watershed: How streamflow relates to minerals and sediment concentrations  

A stream like the Uncompahgre River transports many substances as it flows through a variety of geologic, land use, vegetation, and urban areas. The substances include suspended bits of soil and rock, organic material, nutrients, bacteria, and dissolved and undissolved metals. Suspended material, particularly those containing metals like iron, can produce different colors in streams and degrade water quality. Water quality specialists are particularly interested in the concentrations of bacteria, nutrients, and metals that can be harmful to aquatic life and humans.

What might cause the color in the river on the left side of Figure 1? Does the clear water in Figure 1 indicate the water is healthy and safe to drink and for aquatic organisms.

Figure 1. Left: The Uncompahgre River in Ouray. The clear water comes from Canyon Creek, just upstream, while the yellow-brown water is from the Uncompahgre River above Ouray that has been shunted through the Ouray Hydro Plant and returned to the river. Right: Commodore Gulch is a stream above Red Mountain Creek where the water is clear, but the streambed has been colored by minerals in the water. Melting snow on the bank can deposit soil and rock particles into the stream.

Dissolved and undissolved particles suspended in a stream reach there in a variety of ways that include natural erosion, acid rock drainage from mines, acid runoff over mine waste, runoff from streets and highways, effluent from sewage treatment plants, return flow from irrigation, groundwater from springs and irrigation pumping, and other sources. Both suspended solids and dissolved minerals can produce a variety of colors in the Uncompahgre River, most commonly browns (from soil), yellows, oranges, and reds (from metal compounds). In addition to visual impacts, metals, nutrients, and bacteria can have harmful effects on aquatic life and humans when they reach certain concentrations. Dissolved substances, like metals, can be present even when a stream looks clear and pristine. The concentration of a substance is the amount of its mass in a specified volume of water, so concentrations in a stream will change as the mass entering a stream changes and the volume of water (streamflow) changes.

In the Upper Uncompahgre River Watershed, many of the metals harmful to aquatic life are brought into the river by natural erosion and acidic water drainage from mines and other exposed rock. Most of this metal-contaminated water drains first into Red Mountain Creek, and then into the Uncompahgre River a few miles above Ouray.

The graphs in Figures 2 show how the concentrations of dissolved zinc (top graph) and total dissolved and undissolved iron (bottom graph) in the Uncompahgre change as the river descends, and streamflow increases, to Ridgway Reservoir. The concentration of dissolved zinc starts out very high, and well above the aquatic life standard, in Red Mountain Creek. Then as streamflow increases the zinc concentration decreases steadily to a minimum below Ridgway Reservoir. This pattern suggests Red Mountain Creek is the source of zinc, and its concentration is diluted by water entering the river below the creek.

For total iron Figure 2 also shows very high concentrations in Red Mountain Creek, which cause a large increase in iron concentration in the Uncompahgre from above (1st site on the graph) to below the creek (4th site on the graph). However, unlike zinc, below the creek iron concentrations remain relatively constant or increase slightly as streamflow increases down to the site just above Ridgway Reservoir (7th site on the graph). For the iron concentration to remain relatively constant, the mass of iron entering the river had to increase as streamflow increased.

The concentration-streamflow relationship with dissolved zinc is typical of other dissolved metals in the Uncompahgre Watershed and a similar upstream-to-downstream concentration trend is found throughout the year. However, the pattern for iron is somewhat unique to the high flow months of May through July, when turbulent water can resuspend iron particles in the streambed below Ouray. In the months before and after this period iron concentrations tend to decrease with increasing streamflow, as observed with the dissolved metals.

Figure 2. Top: Graph showing the concentrations of dissolved zinc (gray bars) and streamflow (blue bars) at sites on the Uncompahgre River between Red Mountain Creek and Ridgway Reservoir in May 2012. Red line is the zinc chronic standard for aquatic life. Bottom: Same as graph on top except showing total iron concentrations.

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Please remember to enter the Uncompahgre River Classic peak flow prediction contest by May 1 (for free)!