This project describes the physical and biological characteristics of ephemeral streams of the Sonoran Desert and develops a stream hydrogeomorphic classification incorporating key physical processes (drivers) that create and support riverine and riparian landforms, hydrologic regimes, and biota. A classification was developed on the Yuma Proving Ground (YPG) by analyzing geomorphic, hydrogeologic, hydrologic, and ecological characteristics of ephemeral stream networks. The stream classes and physical drivers were tested to determine which were meaningful for identifying habitat characteristics and riparian biota on the Barry M. Goldwater Range (BMGR).

Three years of field data for representative streams were collected and analyzed to quantify physical patterns and processes, and the factors influencing the distribution of biota. Streamflow, groundwater, and volumetric water content data were analyzed from the full range of stream types to develop a 3-dimensional perspective on surface and ground water interactions, hyporheic zone characteristics, and the seasonality of surface, ground, and soil water that supports riparian biota. Seasonal water relations of dominant riparian trees also were examined to understand how hydrologic variability affects their growth, survival, and reproduction. These data were used to develop a model of hydrologic processes and patterns that support key biota. A factorial experiment was used to determine the physical and biotic limitations to riparian tree reproduction.

These analyses were used to build a climate and hydrology model to understand how climate changes may affect stream flow, subsurface water content, and riparian ecohydrology. The effects of hydrologic and geomorphic alterations associated with roads and other military land uses also were described on fluvial landforms, hydrology, and riparian structure and function. The stream classification provides tools for rapid, long-term and cost effective monitoring and management. The analysis of factors limiting seedling survival can be used to improve revegetation efforts, and could be used as the basis for scaling up to restore entire stream systems.

This project hypothesized that differences in channel planform, lateral confinement, and boundary materials would produce distinctive physical environments and biotic patterns in ephemeral stream networks. Five hydrogeomorphic stream types that are widespread throughout arid regions of North America were outlined: bedrock, bedrock with alluvium, incised alluvium, braided, and piedmont headwater streams. Analyses of physical and biological patterns and processes were framed within this stream typology.

Field data describing geomorphic characteristics and composition of riparian plant communities on 86 stream reaches throughout YPG were collected and analyzed, and the project team verified these observations on 15 stream reaches at BMGR. At a subset of 18 representative study reaches at YPG, the timing, magnitude, and frequency of rainfall and streamflow events were also measured. Eight intensively instrumented study reaches on YPG were used to clarify the interactions of surface water, groundwater, and unsaturated water content. Subsurface alluvial structure and stratigraphy of these sites was also investigated. Ecohydrological relations of riparian trees, including seasonal water sources, patterns of water stress, and sap flow, were documented and related to streamflow and subsurface moisture dynamics. The importance of herbivory, nurse plant shading, and infrequent wet periods on riparian tree seedling survival was determined in a factorial field experiment.

Differences in channel planform, lateral confinement, and boundary materials reflect distinctive geomorphic process domains within ephemeral watersheds, resulting in consistent patterns of channel morphology and hydraulics among the five stream types. Patterns in subsurface stratigraphy and alluvial characteristics were evident at broader spatial scales. Incised alluvium and braided streams that occur within lowland valley settings contain deep (~1 m) active alluvial deposits underlain by consolidated sedimentary strata, while bedrock with alluvium streams in narrow mountain valleys exhibit thinner active alluvium and complex subsurface stratigraphy bounded by bedrock. Bedrock and piedmont headwater channels lack persistent alluvium.

Differences in streamflow responses among stream types correspond to network position and channel boundary materials. Bedrock and piedmont headwater streams, which drain small watersheds with low infiltration rates, experience runoff most frequently. Streamflow occurs less frequently in channels containing unconsolidated alluvial deposits, such as bedrock with alluvium, incised alluvium, and braided streams, where floods occur almost exclusively during intense summer thunderstorms. Direct precipitation from storm events briefly increase water content within the upper 100 cm of alluvial deposits, but deep recharge occurs only after periodic summer floods. In all stream types, streamflow and saturation of shallow alluvium typically lasts for only a few hours, but deeper recharge (2 to 3 m) beneath incised alluvium and braided streams can persist for months after flow events.

The stream types support distinctive riparian plant communities, and differences in community composition closely mirror the distinctive morphological attributes of stream types, highlighting the critical role of geomorphic disturbance in shaping riparian vegetation of arid ephemeral streams. Compositional differences among stream types correspond primarily to variation in channel gradient and width:depth. Within the study area, large woody plants rely primarily on water from near-surface (0 to 50 cm) alluvial sediments, but riparian trees access water from different depths, following seasonal shifts in water availability. All riparian trees access water from greater depths during drought periods, when plant water stress is greatest. In braided and incised alluvium streams, periodic large floods can improve riparian tree water status for several seasons, while limited storage capacity in the shallow substrate of bedrock with alluvium and piedmont headwater streams results in more regular seasonal dynamics. Herbivory by small mammals appears to be the primary limitation to riparian tree establishment within the study area, and significant seedling survival requires protection from herbivores and shading by nurse plants, both of which can be provided by shrubs with dense canopies. Infrequent wet years substantially increase seedling survival rates, and likely influence desert tree population dynamics over timescales of decades to centuries.

Integrated climate and hydrology models suggest that increasing frequency of extreme storm events can result in greater deep recharge, while shallow alluvial water content is relatively insensitive to model parameters. However, many uncertainties remain in predicting the effects of global climate changes on monsoon rainfall patterns that drive ecohydrological dynamics, and resource managers should focus on minimizing hydrologic alterations and maintaining streamflow connectivity to avoid ecological degradation.

This study’s hydrogeomorphic stream classification characterizes variation in fluvial form and function throughout stream networks, and can be used to facilitate management and restoration of ephemeral streams of the southwestern United States. Resource managers could use this conceptual framework to infer basic information on physical and ecological characteristics of fluvial ecosystems, plan the types and locations of land use and human disturbance in order to minimize long-term environmental impacts, and improve the efficiency of resource inventories and mapping projects. These analyses also demonstrated that improved revegetation and stream restoration efforts require providing protection from herbivores and shade, while planting larger seedlings during infrequent wet years maximizes survival rates. This information provides a better understanding for managing and restoring ephemeral streams on southwestern Department of Defense installations, thereby increasing the capacity for sustainable ranges in the context of military training and testing, maintenance of critical riparian habitat for listed and at-risk species, and enabling appropriate restoration of degraded stream systems.