A Little Bit of Rains Makes a Big Change at Story Mill wetlands

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Wetland grasses at Story Mill Community Park

The unusual rains this July have made a dent on the wetlands at the future Story Mill Community Park in north Bozeman. Literally—the thick grasses, usually up to my chest, were beaten down by a hailstorm earlier this week, making my trek through the wetlands on my weekly monitoring circuit somewhat easier.

But the rains have had a much larger impact on something that we can’t see so easily: the groundwater.

Wetlands are places where groundwater—also referred to as the water table or aquifer—is quite high, sometimes at or above ground level. So as the wetland restoration at Story Mill Park progresses, we are interested to see how the groundwater changes. For three years now, GGWC has been measuring groundwater levels in 15 small wells throughout the park.

These measurements provide an interesting look at what’s been happening underground this season. The groundwater levels rose through the month of May as the last of the snow melted off the mountains and spring rains hit town. By the second week of June, groundwater levels were falling as warm and dry weather set in. Bozeman received only 0.6 inches of rain in June, far below the average monthly total of 2.4 inches.

In July, the water has started rising again thanks to recent rain storms. And as of July 29, we have received 1.78 inches of rain. That small amount of rain has made a noticeable difference at the Story Mill wetland. This graph compares water levels in the 15 wells on June 25, July 16, and July 29:

On average, the groundwater rose more than 13 inches between June 25 and July 29. This is a significant increase, especially given that it rained only 1.78 inches during this time.

This demonstrates another important feature of wetlands: their ability to turn surface water to groundwater, and vice-versa. As water levels in Bozeman Creek and the East Gallatin River rise, some of this water is taken into the Story Mill wetland and stored as groundwater.

This is good news in a lot of ways. It means that water that has traveled through urban Bozeman—picking up lawn fertilizer, stormwater, sediment, and more—will be filtered by the wetland, removing some of these pollutants. And if the weather turns dry again this summer, the water that is stored in the wetland can return to the river, providing base flow to keep the aquatic habitat healthy.

The second phase of restoration work will begin next month at Story Mill Community Park to continue enhancing this important ecosystem at the confluence of Bozeman Creek and the East Gallatin River. The Greater Gallatin Watershed Council is proud to partner with The Trust for Public Land on this project.