Modern Terrace Development Near Pyramid Lake, Nevada, and its Geologic Implications

Abstract

The Truckee River rises high in the Sierra Mountains and ends 30 mi (48 km) northeast of Reno, Nevada, where it empties into Pyramid Lake, the largest remnant of Pleistocene Lake Lahontan. Since 1910, the level of Pyramid Lake has declined 80 ft (24 m) due to diversion of water from the Truckee River basin. During this period, the river has formed at least six well-developed, erosional terraces located 1 to 6 mi (1.6 to 10 km) from the present lake and standing between 10 ft (3 m) and 35 ft (11 m) above the present stream. Continuous lake-level measurements and periodic aerial photographs establish that the entire terrace sequence formed within a period of 44 years. The terraces developed during a period of changing geomorphic conditions, a geologic setting different from that normally accepted as controlling the origin of erosional terraces. In contrast to the usual case, the Truckee River terrace sequence formed during a rapidly, although intermittently, declining base level, extreme downcutting, and unstable hydrologic regimes. In years of normal (low) runoff, inflow is not sufficient to maintain the lake, and the stream incises its main channels to keep pace with the lowering base level. During occasional years of high runoff, the entire braided valley is inundated and the valley bottom is quickly broadened by processes of lateral erosion accompanied by minor deposition. A return to normal flow allows lake-level decline and stream incision to resume, isolating the former high-flow flood plain as a terrace. The history of development of this terrace sequence clearly indicates that such terraces should not always be assumed to represent a long-term period of geomorphic equilibrium. Terraces similar in origin to those at Pyramid Lake might be initiated by diverse geologic processes in a variety of settings. Thus, the modern terrace development here has geologic significance. Examples are: (1) the gradual destruction of lakes dammed by moraines or landslides; (2) stream captures of tributaries to the river entering a lake, which effectively reduce the annual inflow; and (3) a change to a more arid climate which upsets the regional hydrologic equilibrium.