9.1: The Sierra Nevada Today

Physiographic Proprieties and Characteristics of the Sierra Nevada

The Sierra Nevada Province is a prominent geological and physiographic region extending approximately 400 miles (644 km) in a north-northwest direction. It is bounded by the Cascade Province to the north, the Basin and Range Province to the east, and the Great Valley Province to the west, with the Garlock Fault forming its southern boundary and separating it from the Mojave Province. This extensive mountain range is characterized by a diverse landscape that includes a mix of rugged peaks, expansive valleys, and dramatic geological features shaped by a long history of tectonic activity.

The Sierra Nevada is primarily composed of intrusive and metamorphic rocks, but it also hosts significant volcanic features that speak to the region's dynamic volcanic past. From ancient lava flows that now form resistant tablelands to striking examples of columnar jointing, the range showcases the results of both ancient and relatively recent volcanic activity. Volcanic domes, remnants of past eruptions, can be found throughout the range, along with deposits from volcanic mudflows, or lahars, which have left their mark on the landscape. The Sierra Nevada's complex geology, diverse topography, and striking natural features make it a key area of interest for understanding the geological history and ongoing processes that have shaped the western United States. This chapter will explore these aspects in detail, highlighting the province's key physiographic properties and characteristics.

Snow and Rain

The Sierra Nevada's climate is profoundly shaped by California's Mediterranean climate, which features wet winters and dry summers. During the fall, winter, and spring months, the range experiences significant precipitation, varying from 510 to 2,030 millimeters (20 to 80 inches), with the majority falling as snow at elevations above 1,800 meters (6,000 feet). This snowfall is a critical component of California's water supply, as the snowpack accumulates during the winter and melts in the spring and summer, feeding rivers and reservoirs that supply water to millions of people in the state. The highest levels of precipitation occur on the central and northern portions of the western slope, particularly between 1,500 and 2,400 meters (5,000 and 8,000 feet) in elevation (see Figure 9.1.1). This phenomenon is driven by orographic lift, where moist air masses are forced upward by the Sierra's towering peaks, cooling and condensing to produce precipitation. However, above 2,400 meters (8,000 feet), precipitation levels diminish as much of the moisture has already been released at lower elevations. This creates a gradient where the western slopes are lush and verdant, while the eastern slopes, lying in the rain shadow of the Sierra crest, are much drier, receiving less than 635 millimeters (25 inches) of precipitation annually.

Figure \(\PageIndex{1}\): As air rises up the windward/western side of the Sierra Nevada mountains, it condenses has its moisture squeezed out. When the air begins to descend the leeward/eastern side of the mountains, it expands and is dry. This work by UC Merced is in the public domain. Access a detailed description.

The summer climate in the Sierra Nevada contrasts sharply with the wetter seasons. Most summer days are marked by dry and warm conditions, typical of the Mediterranean climate. However, the region is not entirely devoid of summer precipitation. Afternoon thunderstorms are relatively common, particularly during the North American Monsoon, which typically occurs in mid to late summer. These storms can be intense, sometimes delivering over an inch of rain in a short period, leading to flash flooding and contributing to the fire risk in the region due to lightning strikes. The Sierra's highest elevations, which boast an alpine climate, experience cooler temperatures year-round and retain snow cover well into the summer months. These alpine zones are characterized by short, cool summers and long, harsh winters, where the snowpack can reach significant depths. The combination of heavy winter snowfall and summer thunderstorms creates a dynamic and varied climate across the Sierra Nevada, influencing the region's ecosystems, hydrology, and fire regimes.

Mountains

The Sierra Nevada Mountains, the most prominent geological feature of the Sierra Nevada Province, are renowned for their dramatic topography and significant elevation changes. This majestic range is predominantly underlain by felsic to intermediate plutonic and metamorphic rocks, which have been exposed and uplifted over millions of years through tectonic processes. The Sierra Nevada is distinguished by its asymmetrical profile (Figure 9.1.2): it gently slopes westward towards California’s Great Valley while its eastern side rises sharply from the floor of the Great Basin, a contrast accentuated by the Sierra Nevada fault system. This fault system marks a significant tectonic boundary that has played a crucial role in shaping the range's steep eastern escarpment.

Figure \(\PageIndex{2}\): The Sierra Nevada mountain range is asymmetric with a shallowly sloped western side and a notably steeper eastern side. This work by the USGS is in the public domain. Access a detailed description.

Elevations within the Sierra Nevada range from approximately 2,700 to 4,300 meters (9,000 to 14,000 feet), with the highest peaks located in the southern part of the range. The most notable of these is Mount Whitney, the tallest peak in the contiguous United States, standing at an elevation of 14,496 feet (4,418 meters). Mount Whitney towers more than 3,048 meters (10,000 feet) above the Owens Valley to the east, creating a striking contrast between the mountain's summit and the valley floor. The southern Sierra Nevada is particularly rugged and elevated, featuring a series of high peaks and steep canyons. This part of the range also exhibits a noticeable westward bend just north of the Garlock Fault, a major geological feature that marks the boundary between the Sierra Nevada and Mojave Desert provinces.

Figure \(\PageIndex{3}\): "Relief Map of the Sierra Nevada" by Cole Heap is licensed under CC BY-NC 4.0 Constructed in a GIS using data from California Open Data Portal. View a detailed description of this image. Access a detailed description.

To the north, the Sierra Nevada gradually decreases in elevation, with peaks becoming less prominent as the range extends towards the Cascade Province (Figure 9.1.3). This gradual reduction in height is accompanied by a change in landscape, with the northern Sierra displaying more gentle topography compared to the rugged terrain of the southern Sierra. The varied elevations and distinct topographical features of the Sierra Nevada not only define its landscape but also influence the region’s climate, hydrology, and ecology, making it a critical area for geological and environmental studies.

Streams and Canyons

The Sierra Nevada Province's asymmetric topography significantly influences its river systems. On the western side of the range, the gentler slope allows for the development of a series of regularly spaced river drainages that flow toward the Pacific Ocean. These rivers, which cut through the mountainous terrain, have carved deep valleys and gorges over millions of years. From north to south, the major rivers include the Feather, Yuba, American, Cosumnes, Mokelumne, Stanislaus, Tuolumne, San Joaquin, Kings, Tule, and Kern. These rivers play a crucial role in the hydrology of California, providing water for agriculture, cities, and ecosystems throughout the Central Valley and beyond.

In contrast, the eastern slope of the Sierra Nevada is much steeper, leading to rivers with high gradients that rapidly descend into the adjacent Owens Valley and other low-lying areas. The stark difference in elevation between the crest of the Sierra and the valley floor creates some of the most dramatic landscapes in the region. The rivers on this side of the range, though fewer in number and shorter in length, are characterized by their fast flow and the deep canyons they have carved into the eastern escarpment. These eastern rivers are critical for the arid regions they drain into, providing essential water resources in an otherwise dry landscape.

The accompanying map (Figure 9.1.4) illustrates the major river systems in California, highlighting the westward-flowing rivers of the Sierra Nevada Province and their contribution to the state's overall water system. The image serves as a visual guide to understanding the distribution and flow patterns of these rivers, emphasizing the importance of the Sierra Nevada's topography in shaping California's hydrology.

Figure \(\PageIndex{4}\): Most of California's rivers originate in the Sierra Nevada province and flow westward feeding into either the Sacramento or San Joaquin rivers. They ultimately flow into the Pacific Ocean through San Francisco Bay. This work by Shannon1 via Wikimedia Commons is in the public domain. Access a detailed description.

References

1. Argus, D.F. and Gordon, R.G. (1991). Current Sierra Nevada-North America motion from very long base-line interferometry: implications for the kinematics of the western United States. Geology, v. 19, p. 1085-1088.
2. Bateman, P.C. (1992). Plutonism in the central part of the Sierra Nevada batholith, California. U.S. Geological Survey Professional Paper, v. 1483.
3. Figueroa, A.M. and Knott, J.R. (2010). Tectonic geomorphology of the southern Sierra Nevada mountains (California): evidence for uplift and basin formation. Geomorphology, v.123.
4. Miller, J. R., & Dunne, T. (1977). Topographic influence on the regional groundwater flow systems of the Sierra Nevada. Journal of Hydrology, 35(1-2), 35-49.
5. Storer, T. I., & Usinger, R. L. (1963). Sierra Nevada Natural History: An Illustrated Handbook. University of California Press.