Weather and Erosion  
Sheep and Taylor Mountains Ancient Oceans

Weather plays a large part in creating the valley and mountains that we see today. Snow, rain, and wind are large factors in the continued erosion, creating deeper mountain canyons, new creeks, wider rivers, filled-in lakes and determining the type of vegetation and animals that can exist here.

The expansive system of shallow lakes, fens, carrs, serpentine river channels,
potholes, marshes, cold mountain streams, riparian shrublands, wet grasslands and warm springs that covers so much of the Centennial Valley floor gives clue to how the mountains continue to erode from weather and movement of water.

Many snow and rain fed springs and streams emanate from the Centennial Mountains and steepen the areas between the glacially eroded mountain peaks by loosening rock, and washing gravel and sedimentation to the valley floor. Large pyramidal shaped alluvial fans test.html at the base of most of these steep canyons of the valley further attest to the continued erosion of these mountains. Many of the fans were formed when the glaciers melted 10,000 years ago and created large floods, but these fans continue to build from mountain streams. Occasional avalanches contribute to the erosion of the mountain faces here as evidenced by the several avalanche chutes visible along South Centennial Road.

Rivers and streams also helped shape the Centennial Valley. The Red Rock River originally drained to the northeast into Madison Valley through a series of lakes that begins at Elk Lake and proceeds to the northeast to the Madison Valley. About 28,000 years ago, this drainage pattern changed as landslides created a large Pleistocene Lake Centennial, in the east Centennial Valley. This filled to over 60 feet higher than the current lakes until it spilled over a natural outlet near the current location of the Lima Dam. The current lakes are mere vestiges of the original lake.

Prevailing weather systems here come from the south and southwest. The east-west trending nature of the Centennial Mountains cause the air masses to rise as these storms hit the back side of the mountains and creates a rain shadow effect and a high desert environment in the Centennial Valley. The Centennial Valley is distinguished by extensive sagebrush (Artemisia ) communities as evidence of this high desert environment. Even with this, the average annual rainfall is close to 22 inches, double some of the other mountain valleys in Montana. The nature of the geography here allows cool air to pool and keeps the valley quite cold, esp. in winter. Winter snows last well into May providing summer moisture to the many plants and animals. Over 150 inches of snow can accumulate on the top of the mountains here before spring thaw. Afternoon thundershowers are typical in the summer months, keeping the streams flowing and the grasses green through most of the summer months. Erosion is year round.

In some years of excessive rain or snow, or early warming, excessive snow melt causes creeks to overflow. (See this example in 2011 of Hell-roaring Creek overflow). These sometimes create new paths for the creeks and accelerate the erosion of the valley and mountain watershed areas. Human intervention in the valley has tended to limit the creation of multiple creek paths due to the creation of roads, drainage pipes, and ditches that constrain the water flow to a specific path.

Snow melt on the valley floor during May and early June can keep a large portion of the Refuge wet or under water. This contributes to unique grasses and plants and provides habitat for water fowl, moose and other mammals late into the spring and early summer.

Throughout the past few thousand years, as the lakes have waned in size due to various factors, winds from the south have pushed sand from dry lake bottoms and dry creeks up along the northern hills to create many sand dunes. These sand dunes contribute to the geologic and ecological uniqueness of the Centennal Valley. The North Valley Road travels through several areas that are fine sand.Google Interactive Viewer Most of the dunes are now covered with sagebrush and grasses and aren't readily identified as sand dunes. However, below this vegetation is sand. Rare plants thrive in this unique ecosystem such as the Painted Milkvetch.

Another cause of mountain erosion is gravitational. Rocks, and debris are constantly being brought DOWN by gravity, often lubricated by rains, earthquakes, and wind. Sometimes large boulders will crash hundreds of feet into the valley. In the extreme, landslides make large changes to the face of the mountains. These can occur when underwater streams loosen the base of the soil on a steep mountain slope.

The Centennial Valley contains the largest wetland complex in the Greater Yellowstone Ecosystem. Many streams flow from the northern slope of the Centennials mountains and the southern slopes of the Gravelly Range and others are from spring fed locations. Watersheds in these mountain ranges create the major year-round streams, including Tom Creek, Hell-roaring Creek, Odell Creek, and Long Creek. Some are spring fed like Elk Springs creek, Picnic Springs and many others. All of these flow into Upper or Lower Red Rock Lake or Red Rock Creek, which flows into Red Rock River, a tributary of the Beaverhead River, thence to the Big Hole River, the Jefferson River and eventually flows into the Missouri and Mississippi Rivers. The water system here is at the origin of the longest river system in North America and the fourth longest in the world, spanning over 3745 miles from Bower Springs (below Mount Jefferson in the Centennial Mountains) to the mouth of the Mississippi River.
The climate of greater southwestern Montana can be described as semi-arid and tending toward Continental with a strong winter-summer temperature contrast; however, the numerous mountain ranges generate local distinctions and peculiarities. For instance, the east-west orientation of the Centennial Mountain range positions it to intercept cells of moist air that originate in the Gulf of Mexico and “drift” northward in mid to late summer. These cells are the source of relatively predictable afternoon thundershowers that can be quite intense and can cause the mountain meadows to remain green long into the growing season. Average annual precipitation at Lakeview (6,700 ft) at the very base of the range is 21.2 inches -- quite high for a valley location (compare to Wisdom, Montana another valley location at 6,100 ft. elevation, which receives only 11.8 inches annually). Precipitation probably exceeds 50 inches near the crest of the Centennial Mountains. Precipitation is relatively evenly distributed throughout the year with the exception of a spring bulge in May and June, a period typically receiving a little less than 1/3 of the average annual precipitation. This precipitation bulge is typical for western Montana’s mountainous areas. Not uncommonly, snowfall occurs every month of the year and can accumulate by the end of winter to depths of 150 inches. The average July maximum temperature is 76°F; the warmest recorded summer day has never exceeded 94 degrees. The average January minimum is -1° F; the coldest day in December, January, and February can be lower than -40º F with an all-time record low of -49º F. The yearly mean temperature is only 35º F, the lowest among recording stations in Montana and lower even than Montana’s well-known cold spot, Wisdom (in the Big Hole Valley). Though frost can occur in every month, the frost-free season averages about 51 days, from mid-June to mid-August. These figures alone suggest that the valley is a very cold environment, as could be predicted from its high elevation. This is exacerbated by the narrowness of the valley, the high mountain ranges surrounding it, and hills of the Alaska Basin just to the east, which impede airflow. These factors cause deep ponding of cold air in the Valley and the persistence of snowpack well into spring months. The fact that subalpine forest extends nearly to the valley floor on the north flank of the Centennial Mountains reflects the relatively cold temperature regime of this valley as well as its high precipitation.