This geocache is part of a planned series "South Hills Stormwater", that will take geocachers on an interactive and educational tour of the stormwater management features in southeastern Missoula, hiding beneath our feet. As a hydrogeologist, I have always been fascinated with the ways that humans interact with and modify the water cycle. The fate of Pattee Creek, as it travels down Pattee Canyon, and then mysteriously disappears without a trace near Bancroft Ponds inspired me to dig into Missoula's stormwater system and hide this series of geocaches. For more reading, please see the references at the end of the description.
A Fork in Pattee Creek
Before the urbanization of southern Missoula in the 1950s, Pattee Creek flowed across the Missoula Valley in a complex network of braided channels that infiltrated into the ground surface at the valley floor. The stream likely changed paths from year to year, migrating freely across the valley. Shortly after permanent settlement, irrigation ditches were constructed to utilize the water flowing through Pattee Creek. The Creek was further channelized between the 1950s and 1990s to make way for more urban development. Because the natural floodplain was paved over, Pattee Creek would regularly overtop its banks and spill into the streets of southern Missoula. After several flooding events in the 1970s and 1980s, exacerbated by events like wildfires and ash fall from the Mt. St. Helens eruption, the City of Missoula and Missoula County constructed the Missoula South Hills Storm Drainage System, a complex sytem of pipes and detention basins designed to handle a 100-year flood. Most sections of Pattee Creek were routed underground, preserving just a small section of open channel.
This geocache is hidden just upstream of where Pattee Creek enters a stormwater settling basin. Please be careful during high water or icy winter conditions not to slip into the creek. As Pattee Creek enters the pond, the velocity decreases, and suspended sediment carried by the creek drops out. This helps to keep downstream ditches and pipes clear of debris and prevent flooding. The pond also acts as a bioremediation system to remove excess nutrients that are picked up by the creek from fertilizers, septic system effluent, and pet waste. From the stormwater basin, Pattee Creek forks; part of Pattee Creek is conveyed under Higgins Ave to the open ditch along Pattee Creek Drive, while the other part is conveyed by the 100-year stormwater gravity main (GCBKQX4), far beneath the streets.
Bonus content: As you approach GZ, you may notice a concrete pipe discharging water into Pattee Creek. The source of this water is a gravity-fed stormwater main that collects water from homes along Whitaker Driver, and discharges it directly to Pattee Creek without treatment. See the Missoula stormwater GIS portal link below for an interactive map!
To continue this geo-series, you can walk along the above-ground portion of Pattee Creek towards GCBHNP2, peek into the 100-year storm drain at GCBKRVW, or hike through the upper Pattee Cree watershed at GCBKQX4.
I will update this text as more geocaches in this series are added.
Happy Hunting!
References and Further Reading
Butterfield, Carver D. (2024). From Creek to Ditch and Back Again: The History and Future of Pattee Creek, Missoula, Montana. University of Montana. Accessed online at: https://scholarworks.umt.edu/cgi/viewcontent.cgi?article=13460&context=etd.
Missoula Montana Stormwater Portal: https://www.ci.missoula.mt.us/2138/Stormwater
Missoula Montana Interactive Stormwater GIS Portal: https://www.arcgis.com/home/item.html?id=8a179071f15848cf961d3586fc6bf4d5