Chapter 2: The Gallatin Watershed

What is the Gallatin Watershed?

Chances are that wherever you live in the Gallatin Valley, a stream, river, or irrigation ditch is less than a few hundred feet from your door. This abundance of free-flowing water gives the Gallatin Valley its unique character. A “watershed” includes the land that water flows over or under from its highest points on hilltops and mountains to its lowest points along streams, rivers or lakes. The Gallatin Watershed is composed of the streams and underground water that flows to the Gallatin River. Nested within the Gallatin Watershed are the smaller watersheds of its tributaries, such as the Taylor Fork Watershed and the Bozeman Creek Watershed. How each of us uses the soil, water, plants, and animals within the watershed affects what happens not only in our watershed but also in larger watersheds downstream. We are all part of the Gallatin Watershed community

Sources of Water

The primary source of water for streams, springs, and wetlands in the Gallatin Watershed comes in the form of mountain snowpack. The headwaters of the watershed, in the Gallatin and Madison Ranges, receive nearly 300 inches per year of snow on average. This equals approximately 50 inches of rain. In contrast, the lower watershed at Logan may receive fewer than 12 inches of rain per year–a near desert by comparison.

A Cross-Section of the Gallatin Watershed

Upland Area Soil: Varied geologic rock types consisting of gneiss, limestone, shale, sandstone and volcanic rocks, covered with a thin mantle of soil. Vegetation: Douglas fir, shrub community at higher elevations, subalpine fir, shrub community at lower elevations.	More than 6 Feet to Water Table Soil: Moderately fine and fine textured loess and alluvial soils. Vegetation: Grass, forb, shrub community.	More than 6 Feet to Water Table Soil: Medium to moderately fine-textured alluvial soils varying in depth over sand and gravels. Vegetation: Grass, forb, shrub community.	Less than 6 Feet to Water Table Soil: Medium to moderately fine-textured alluvial soils varying in depth over sand and gravels. Vegetation: Grass, sedge, shrub community.	Gallatin River

Diagram by Tony Rolfes, NRCS.

Geology and Soils

The main stem of the Gallatin River originates in the Madison and Gallatin Ranges. These mountains are composed primarily of “basement rocks” of gneiss and schist, covered in places by a thin layer of rocks formed from the sediments that sank to the bottom of an ancient shallow sea that covered Montana from 570 to 65 million years ago. The East Gallatin originates in the Bridger Range, which is also comprised of basement rocks overlain by younger sedimentary formations. Where the streams leave the mountains, the valley widens into a large basin filled with sediments, some more than 6,000 feet deep east of Bozeman Hot Springs. These sediments, laid down during the Tertiary period (from 65 to 2.5 million years ago), vary in type, but are mostly rocks made from sand and silt. On top of these sediments are up to 150 feet of gravel, sand, silt, and clay washed from streams. These deposits are called alluvium. More recent alluvial deposits compose the next and last layer of fill, which covers half the valley. These deposits extend in large fans from the source streams where they break out of the mountains. The largest fan is at the base of the Gallatin Range south of Bozeman. This very recent or Quaternary alluvium is mostly comprised of pebbles and gravel. Its thickness ranges from 70 feet at Bozeman Hot Springs to 800 feet near Belgrade. The Quaternary alluvium that covers most of the central valley is important, because it is some of the most permeable material in the valley and the most reliable source of groundwater.

Soils in the watershed are a product of the rocks that form them. Coarse-textured rocks like gneiss, schist, and sandstone typically form gravelly and sandy soils, while sedimentary rocks like shale and limestone form clayey soils. Much of the stream sediment carried in the spring runoff comes from areas of fine-textured rocks and clay-rich soils. Many landslides and earthflows in the watershed are associated with fine-textured soils, and contribute high amounts of sediment to the Gallatin watershed. In contrast, soils formed from coarse-textured rocks usually allow passage of water at a high rate and are highly erodible. Valley soils are usually formed from either fine-textured sediments, wind-blown silt deposits, or stream alluvial deposits. Many soils in the central valley are saturated with water for much of the growing season. These “hydric” soils, unless drained, favor the kinds of water-loving plants that grow under conditions of reduced oxygen. Depending on where you live, management concerns may include high water tables, unstable or erosive stream banks, or shallow depths to sand and gravel, all of which can affect waters in the Gallatin Watershed.

A "hydrograph" shows changes in annual flows, here for the Gallatin River near Gallatin Gateway.

Source: USGS

Text description of changes in annual flows for the Gallatin River. Includes a larger image of the hydrograph.

Hydrology of the Watershed

Surface water

Water held as snowpack in the Gallatin and Madison Ranges typically contributes to peak flows in lower watershed streams and rivers in May and June. While this is also true of streams from the Bridger Mountains, a higher percentage of runoff from the Gallatin and Madison Ranges occurs later in the summer than from Bridger Mountain streams. Because of the higher snowpack, these streams are more reliable sources for late-summer irrigation. Of the total annual yield of water from the tributaries of the Gallatin, about 14 percent comes from streams originating in the Bridger Mountains (Rocky, Bridger, Middle, Cottonwood, Reese, and Dry Creeks); 14 percent comes from streams originating in the north Gallatin Range (Hyalite, Sourdough, Story, South Cottonwood, and Big Bear Creeks); 11 percent comes from tributaries of the north Madison Range (Spanish, Hell Roaring, and Fish Creeks); and the rest (61 percent) comes from the South Gallatin and Madison Ranges.

Groundwater

Groundwater is an important source of water for drinking and irrigation in the Gallatin Valley. Valley fill deposits are the primary aquifer from which most of the domestic ground water drinking water supply is drawn. About 2,500 acres of the watershed are irrigated with groundwater. Groundwater flow in the valley is generally from the east and southeast to the northwest, where the Gallatin River exits the valley at Logan. Depth to groundwater varies from as close to the surface as 3 feet in the central valley to 460 feet in the Camp Creek Hills. Groundwater depth is greatly influenced by irrigation practices. In years when less irrigation water is used, depth to groundwater drops in areas traditionally irrigated and rises in areas where irrigation is not practiced. Flood irrigation and leaky ditches can contribute to higher water tables during the growing season, after spring runoff has occurred. Groundwater depth fluctuates seasonally as well, with the lowest depths occurring in January and February. Periods when groundwater recharge occurs coincide with peak flows from surface water.

Depth to groundwater can vary throughout the season.

Source: Slagle 1995.

Text description of variations in depth to groundwater. Includes a larger image of the graph.

Real-time hydrographs for the Gallatin River are available from the U.S. Geological Survey web site. Data from 56 other gauging stations in the watershed are available at the National Water Information System.

Water resource maps are available from the Natural Resources Inventory System, Montana State Library.

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