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Water Wisdom: Will deep snows equal deep flows?

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By David Tucker EBS CONTRIBUTOR

Recently, winter stormed back onto the southwest Montana landscape. A March 24 storm left almost a foot of snow on the slopes high above Big Sky, with a few inches dusting the meadows and canyons closer to the Gallatin River. On the morning of March 25, our watershed’s snowpack measured 116 percent of normal, according to the Natural Resource Conservation Service.

That number bodes well for this year’s water supply, as a deep mountain snowpack tends to guarantee the release of cold, fresh water through the summer and early fall. When spring temps warm, snow melts and runs off as surface flows in our creeks, streams and rivers. This melt also seeps into the groundwater, recharging the aquifers that feed our water supply. As of this moment, it appears 2020 will be a good year for water.

However, it has become increasingly hard to use a deep winter snowpack to predict the behavior and availability of water throughout spring and summer, due in large part to climate change-related variability.

According to the Montana Climate Assessment published in 2017, spring runoff is occurring earlier and faster than normal, meaning water that previously remained in the watershed later in the season as snow is now melting and running off downstream earlier. This limits late-season river flows, and combined with warmer average temps, creates conditions harmful to aquatic life while contributing to unwanted levels of algae like we saw in 2018. This could also lead to a lack of water for human use, a condition that is exacerbated by growing demand from commercial and residential development.

This shift toward earlier runoff is related to warming trends associated with human-caused climate change, and will likely become more pronounced as time goes on. As the MCA suggests, Montana temperatures are expected to rise by 4.5-6 degrees F by mid-century, and warming is expected across all seasons.

While Montana is expected to warm, the state is also expected to experience more precipitation, a trend that seems positive for water supply. However, while scientists predict increased precipitation in fall, winter and spring, the warmer temperatures mean less of this precipitation will fall as snow.

Snow is critical to Montana’s water supply because it is released slowly and consistently across the landscape. When water falls as rain, it leaves the ecosystem faster, meaning less is available later in the summer when precipitation amounts are lower. On top of this is the expectation that summer precipitation is expected to decrease further, resulting in less water making its way into our groundwater aquifers.

Adding to the water-retention challenges associated with a warming climate are the human-altered landscapes that can accelerate runoff. Sidewalks, driveways, roads and parking lots contribute to a more rapid runoff, as water that hits these surfaces has no chance to seep into the groundwater aquifers. Instead, the majority of this water, be it rain or snowmelt, rushes straight to the river and downstream.

Mitigating this otherwise negative impact is possible by way of stormwater management infrastructure like the conservation garden planted at the Lone Peak High School parking lot, designed and funded by the Gallatin River Task Force. The lot funnel’s water into a vegetated area. Here, the flow is slowed and water can seep into the groundwater, and it is better filtered before dumping into the Gallatin River.

While building this type of resiliency into Big Sky’s water supply is a step in the right direction, and a necessary goal regardless of climate change projections, more needs to be done to combat warming trends and shifting precipitation patterns. If warming continues, there simply won’t be enough water in the watershed for healthy ecosystem function or human use.

David Tucker is the communications manager for the Gallatin River Task Force.

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