The Helena Valley is a topographic and structural intermontane basin in Lewis and Clark County, Montana. The valley encompasses the city of Helena and surrounding communities, residences, and farmland.
Geography
Surrounding formations The Helena Valley is surrounded by various geographical areas, including the Northern Rocky Mountains, the Continental Divide, the Boulder Batholith formation, Gates of the Mountains, Adel Mountains, Big Belt Mountains, and the Elkhorn Mountains. Additionally, various rock formations and types can be found in the area including Pilgrim Limestone, Park Shale, Meagher Limestone, Wolsey Shale, Flathead Sandstone, Helena Dolomite, Jeferson Dolomite, Three Forks Shale, Mississippian Madison, Spokane Shale, and other rocks, but Cretaceous, volcanic, and other rocks from similar time periods generally make up the area. However, major collections of rock formations occur more often in the mountains and other formations surrounding the valley.
Geographic formation of the Helena Valley Long before the Helena Valley, the Boulder batholith stood where Helena sits. It is said that the rocks at the time were around 80 million years old. Additionally, other formations stood, including granitic stocks. Then, around 65 million years ago, an east–west expansion pulled the mountains out, which allowed for the forming of the valley around 50 million years ago. Afterwards, the Helena Valley fault caused the northeast corner of the valley to subside. In addition to subsiding, the shift caused bedrock to be deposited into the soil, creating a bedrock layer. Today, topographic data of the region show that this formation shift is now 6,000 feet below the surface of Lake Helena. More recently, around 5 million years ago, seismic events caused erosional surfaces, deposits coarse gravel, and bedrock erosions. Even more recently, from 100,000 to 150,000 years ago, glaciers were formed and drained, causing creeks such as Tenmile Creek to be formed. Such creeks deposited fine-grained deposits during their creation. Today, nearby mountains to the valley are eroding and the valley is slowly filling with materials.
Geographic faults and seismic formations of the Helena Valley
Beneath Helena sits a major seismic fault-the Intermountain Seismic Belt. Although the belt typically produces small and minor earthquakes, the zone expands from Arizona to Montana and has been known to produce moderate to severe earthquakes before, including the 1935 Helena Earthquake. In addition, the major belt, Helena also has a small seismic belt called the Helena Valley Fault System, which is a collection of small faults and cracks under Helena. The system also typically produces little earthquakes, and most are minor in size. With the Rocky Mountains surrounding the valley, small faults generally are forming all the time from tectonic movement under the mountains, which can cause small seismic activity from time to time.
Geographic activity and events in the Helena Valley
Due to the various rock formations, complex geographic history of the area, and several fault lines beneath, the valley has experienced many seismic events, with smaller ones being more frequent although larger earthquakes have happened before. According to Homefacts and the United States Geological Service, Helena has experienced 239 earthquakes within a 30-mile radius since 1931 and has a 51.58% possibility of another one 5.0+ magnitude in the next 50 years. Over history, Helena has experienced several minor to severe earthquakes. In 1935, the "Big Shake" occurred, a 6.2 magnitude earthquake dubbed the 5th most destructive earthquake in U.S. history. The area, although did not form the quakes, also experienced the Hebgen and Clarkston Valley Earthquakes, both above 6.0 magnitude. Most recently, a 2.7 earthquake occurred on March 9, 2025, near the city.
History
Municipalities The Helena Valley contains two cities: Helena East Helena and seven census-designated places: Helena Valley Northwest Helena Valley Northeast Helena Valley West Central Helena Valley Southeast Helena West Side Montana City Lakeside
Natural features
Lakes Aside from mountains and various rock formations, the Helena Valley contains many other natural features. To the northeast is Upper Holter Lake, also known as the Gates of the Mountains. The lake offers boat tours, hiking, camping, and other recreational opportunities. Also to the northeast is Lake Helena and Hauser Lake, which both offer plenty of recreational activities as well. Although not natural, the Helena Valley Reservoir is a reservoir on the east edge of the Helena Valley. The reservoir offers the same recreational opportunities as most lakes in the area. Additionally, although not in the valley, Canyon Ferry Lake is a major recreational spot to the southeast of the valley.
Rivers and streams Supplying the water for Hauser Lake, the Holter Lakes, and Canyon Ferry Lake is the Missouri River. The Missouri River runs on the east edge of the Helena Valley. The river also receives water from Lake Helena, the Helena Valley Reservoir, and other small water features that flow through the valley. Also on the east edge of the Helena Valley is Prickly Pear Creek. The creek flows through East Helena and Helena Lake before emptying into the Missouri River. On the west side of the Helena Valley is Tenmile Creek, which flows through the edge of Helena and other communities before emptying into Prickly Pear Creek. Many other smaller creeks are in the Helena Valley, but are often on private land and offer little recreational access.
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