Voice of the River

Stormwater Management in Big Sky

Recently, I stopped along the Gallatin to enjoy a peaceful moment at the river’s edge. Pulling into the access site adjacent to the Hwy. 191 stoplight, I expected an idyllic scene of slow-moving, crystal clear water. What I got was the opposite.

Originating from sources unknown was a thick stream of chocolate-milk-colored liquid. Fearing the worst, I assessed the situation to confirm that it wasn’t raw sewage fouling the Gallatin.

The turbid water was in fact running off from a nearby parking lot. Plowed snowbanks were melting, and the melt water was mixing with newly exposed dirt. The result was a sediment-filled flow headed to the river.

This is “stormwater runoff,” and it is a growing problem in Big Sky. The more parking lots, roads, roofs, and sidewalks constructed, the bigger the problem becomes.

These impervious surfaces hasten the flow of water during spring melt and summer rain. Sediment-filled water flows rapidly down streets and into streams, carrying along with it oil, greases, and harmful chemicals. The Middle and South forks of the West Fork, and the West Fork of the Gallatin itself, are currently designated “impaired” by Montana DEQ standards in terms of sediment load. If we don’t do something about it, the main stem of the Gallatin could be next.

Pollution is just one part of this problem. When snow melts or rain falls and there is vegetated soil for it to seep through, water can make its way into our groundwater, recharging local aquifers. These aquifers release groundwater throughout the summer and must be fully recharged to provide sufficient flows for late summer and early fall. Without those pervious vegetated surfaces, the stormwater rapidly runs off, leaving the upper watershed too soon. This leaves the river low and hot come August and September. Insufficient flows and high-water temperatures contribute to excess algae growth, such as we saw in August 2018.

So, what’s the solution?

Land developers in Big Sky should implement stormwater infrastructure and Best Management Practices (BMPs) for both the construction and post-construction phases, and for both detention and treatment. “Big Sky has no coordinated stormwater runoff management,” says Kristin Gardner, executive director of the Gallatin River Task Force, “which I believe is a huge issue.”

While some new development of more than one acre is regulated for construction-phase stormwater management by Montana DEQ, “historical infrastructure likely has little to no detention or treatment infrastructure,” according to Mace Mangold, senior project engineer with WGM Group. Furthermore, Mace explains, Big Sky is not “within a regulated municipality,” meaning the town’s unincorporated status leaves the Gallatin more susceptible to pollutants from inadequately regulated stormwater runoff management.

Structures like detention ponds, infiltration basins and vegetative implementations reduce and treat stormwater while delivering social, environmental and economic benefits. Grassed swales (ditches) and filter strips can be strategically placed in residential areas to help reduce stormwater runoff through infiltration and storage, while keeping neighborhoods and commercial districts visually appealing.

BMPs and stormwater infrastructure would be most useful in the Mountain Village and Meadow Village areas because of their dense impervious-surface compositions, but they should also be implemented at large residential properties, hotels, and base-lodge facilities.

On the same day pollutants poured into the Gallatin from the poorly managed parking lot mentioned earlier, sediment-filled runoff was being captured and filtered at the Ophir School’s Stormwater Conservation Garden. Just a few miles down the road, a problem was being solved. This is the approach Big Sky must take as a community if we are going to keep the Upper Gallatin healthy, clear, and cold for future generations.

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