Geomorphology, Hydrology, and Ecology of Great Basin Meadow
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Chapter 2: Controls on Meadow Distribution and Characteristics Dru Germanoski, Jerry R. Miller, and Mark L. Lord Introduction precipitation values are recorded in the central and southern Toiyabe and southern Toquima Mountains (fig. 1.4) and, as eadow complexes are located in distinct geomorphic expected, meadow complexes are abundant in those areas. Mand hydrologic settings that allow groundwater to be In contrast, basins located in sections of mountain ranges at or near the ground surface during at least part of the year. that do not have significant area at high elevation (greater Meadows are manifestations of the subsurface flow system, than approximately 2500 m) and that are characterized by and their distribution is controlled by factors that cause lo- limited annual precipitation contain relatively few meadows. calized zones of groundwater discharge. Knowledge of the For example, much of the Shoshone Range and the north- factors that serve as controls on groundwater discharge and ern and southernmost portions of the Toiyabe, Toquima, and formation of meadow complexes is necessary to understand Monitor Ranges are lower in elevation and receive less pre- why meadows occur where they do and how anthropogenic cipitation. Therefore, meadow complexes are uncommon in activities might affect their persistence and ecological condi- these regions. Analyses of color-enhanced satellite images tion. In this chapter, we examine physical factors that lead to of the region reveal that meadow complexes also are uncom- the formation of meadow complexes in upland watersheds mon in low-elevation mountain ranges in the surrounding of the central Great Basin. We then describe variations in area, including the Pancake Range, Antelope Range, the meadow characteristics across the region and parameters Park Range, and Simpson Park Mountains. that produce differences in the nature of meadow complexes at basin scales. In subsequent chapters, we examine the geo- Geologic, Stratigraphic, and Hydrologic morphic, hydrologic, and vegetation processes that operate Controls within individual wet meadow complexes. Groundwater along riparian corridors of most basins is Factors Affecting Meadow typically located well below the ground surface (usually greater than 3 to 5 m), even when an ample supply of ground- Distribution and Development water would be expected based on the watershed’s elevation and precipitation. Thus, meadow complexes represent an Precipitation anomalous condition where groundwater levels are elevated compared to adjacent upstream and downstream reaches of Wet meadows require a reliable supply of groundwater the riparian corridor. These spatial changes in subsurface that supports high water tables year after year and that sus- water levels raise two important, interrelated questions: tains the phreatophytic vegetation characteristic of these (1) Is the water that is found within meadows entirely de- discrete ecosystems (Castelli and others 2000). The ultimate rived from its down-valley movement through valley fill? supply of this water is precipitation. Annual precipitation and (2) Are other factors causing groundwater levels to rise varies from 150 mm/year to approximately 700 mm/year locally? The meadows may be products of localized inputs in the study area (Oregon Climate Service 2009) and falls of water to the valley that raise water levels, of changes in mostly as snow in winter. Observed spatial variations in hydrologic flow conditions that produce upwelling, or of precipitation closely parallel changes in elevation (figs.1.2 some combination of the two. The answers to these ques- and 1.4). Watersheds that have a significant portion of their tions require an understanding of the geologic, stratigraphic, drainage area located at high elevations should be able to and geomorphic settings of meadow complexes. capture more rain and snow and, therefore, have more avail- History and Timing of Sedimentation Events. The geo- able water. In fact, it has been demonstrated that stream morphic characteristics of the valley bottoms, including discharge and flow duration are proportional to and can be those in the vicinity of meadow complexes, largely reflect estimated by the size of the drainage basin and the percent- erosional and depositional events that occurred during the age of the drainage basin that is located at elevations above late Holocene (approximately 4500 years BP to the pres- 3050 m (Hess 2002). ent). The geomorphic history of the basins was documented The highest elevations within the six mountain ranges primarily by examining and dating alluvial stratigraphic de- in the study area (Shoshone, Toiyabe, Toquima, Monitor, posits in approximately 30 upland watersheds spread across Roberts, and Hot Creek Ranges) occur in the central and the central Great Basin (Miller and others 2001, 2004). The southern portions of the Toiyabe, southern Toquima, and most detailed studies occurred in the Big Creek, Barley, central Monitor Ranges (fig. 1.2). The highest annual USDA Forest Service Gen. Tech. Rep. RMRS-GTR-258. 2011. 11 Kingston, San Juan, Cottonwood, and Indian Valley basins in deposits 0.5 to 2 m thick. In addition, stratigraphic profiles (fig 1.7). Results indicated that while sediment was always revealed that sediment that was eroded from the hillslopes being produced, transported, stored, and exported from was routed through low-order tributaries to the side-valley the upland basins, significant temporal variations in sedi- fans causing fan progradation. Fan sediments were then ment production and transport dynamics occurred during eroded and routed down higher-order channels and axial the late Holocene often in response to discrete changes in valleys resulting in contemporaneous deposition along climate. During the Neoglacial (4500 to 2500 years BP), a major trunk streams. The process created an interfingering period of cooler, moister conditions and relatively high veg- relationship between fan and axial valley deposits (fig. 2.1) etation abundance, the landscape was relatively stable and (Miller and others 2001). This alternating sequence of de- little sediment was transported to alluvial fans located at the position and erosion allowed much of the coarser sediment mouth of tributaries (called side-valley alluvial fans) or to (cobbles and boulders) to be stored in side-valley fans and the alluvial valley floor (Chambers and others 1998; Miller more of the finer-grained materials (pebbles, sand, silt, and and others 2001). As a result, soils formed in the stable val- clay) to be redeposited along the axial drainage system. ley fill and alluvial fan deposits (Miller and others 2001). A product of fan progradation into the axial valleys was Landscape stability changed, however, with the onset of the a reduction of channel gradients immediately upstream. The post-Neoglacial drought that began in this area approximate- reduced gradients appear to have further promoted accumu- ly 2600 years BP. The post-Neoglacial drought was a time lation of sediment along the axial valley (fig. 2.2). In some of relative aridity, during which upland vegetation changed cases, radial fan profiles indicate that fan deposits traversed substantially toward dryland assemblages. Available paleo- the entire width of the valley, effectively blocking down- ecological data (primarily from woodrat middens) indicate valley movement of water and sediment. Although these that the number of plant taxa present decreased from a rel- blockages were probably breached rapidly, they may have atively high level to the lowest levels observed during the promoted sediment aggradation temporarily by reducing Holocene (Miller and others 2001; Tausch and others 2004). downstream flow velocities. The decrease in vegetation cover apparently led to erosional Since about 1980 years BP, the stream systems through- stripping of hillslope sediment when infrequent but intense out the region have undergone periods of stability that were rainfall events occurred. The eroded sediment was redepos- separated by episodes of channel incision. Interestingly, in- ited on side-valley alluvial fans and throughout the valley cision appears to have begun prior to the end of more typical network from approximately 2580+70 to 1980+60 years BP. moisture conditions during the Holocene (Tausch and others A high frequency of wildfires also may have accompanied 2004). Miller and others (2001, 2004) argue that the onset of the drought, as indicated by a large amount of charcoal in incision was related to depletion of fine-grained sediment on alluvial deposits that were created during the time period. hillslopes and reduced sediment loading to the axial valley, Presumably, destruction of vegetation by wildfires facili- an argument that is supported by a general increase in grain tated soil erosion and sediment accumulation in the valleys. size of the valley-fill deposits through time. The inability of The sediment that was generated during the post- the current hydrologic regime to move sediment from the Neoglacial drought buried the soils that had developed in hillslopes likely produced the current tendency for modern earlier deposits. Thus, a paleosol dated to approximately channels to incise in response to natural or anthropogenic 3500 years BP can be found at the base of most post- disturbances. Neoglacial deposits throughout the region and marks the The depositional history of the upland basins led to a beginning of widespread sedimentation. Stratigraphic data rather simple stratigraphic sequence