Duncan T. Patten

RIPARIAN VEGETATION RESPONSE TO ALTERED DISTURBANCE AND STRESS REGIMES

River damming and flow regulation can alter disturbance and stress regimes that structure riparian ecosystems. We studied the Bill Williams River in western Arizona, USA, to understand dam-induced changes in channel width and in the areal extent, structure, species composition, and dynamics of woody riparian vegetation. We conducted parallel studies along a reference system, the Santa Maria River, an unregulated major tributary of the Bill Williams River. Flood magnitude on the Bill Williams River has been dramatically reduced since the closure of Alamo Dam in 1968: the 10-yr recurrence interval flood in the pre-dam era was 1397 m3/s vs. 148 m3/s post-dam. Post-dam average annual flows were higher due to increased precipitation in a few years, but increases in post-dam May–September flows are largely attributable to dam operation. An analysis of a time series of aerial photographs showed that channels along the Bill Williams River narrowed an average of 111 m (71%) between 1953 and 1987, with most narrowi...

Riparian ecosytems of semi-arid North America: Diversity and human impacts

Riparian ecosystems in the semi-arid West of North America are diverse but have many similarities. The mountainous landscape with wide range of latitude, longitude, and elevation offers diverse opportunities for streamside vegetation. All riparian ecosystems in the region are dependent on supplemental water, usually from the shallow, valley alluvial aquifer. Western riparian ecosystems provide several ecological services. They stabilize streambanks, trap sediment, improve water quality, and help control or modulate hydrologic processes. They function as habitat for many western animal species, serving as a small mesic island or strip within an arid landscape. They also serve as recreational sites for humans. Riparian systems are controlled by interacting hydrologic and geomorphic processes. Floods may alter river channel characteristics and the extent of riparian vegetation while enhancing recruitment of riparian species and recharging the alluvial water table. Geomorphic features, such as canyons and valleys, control the size of the riparian zone, as well as depth of the water table. Driving variables may differ from north to south, especially hydrology. For example, northern riparian zones are influenced by ice scour, while southern zones often have flash floods. Riparian systems occur along spatial and temporal gradients. Along elevational gradients, riparian vegetation may change from simple deciduous forests to mixed deciduous to coniferous and possibly alpine wetlands. Differences among channel, terrace, and upland plant communities decrease with increasing clevation as moisture stress decreases. Temporal gradients occur within a location in the riparian zone as early pioneer communities such as cottonwood/willow give way to late successional communities such as mesquite or sagebrush, often a consequence of sediment accumulation. Many similarities among western riparian ecosystems exist because several dominant genera (e.g.,Populus) are common throughout the West, and many geomorphic and hydrologic processes that influence riparian establishment are similar. Western riparian ecosystems have been greatly altered by human activity. Major factors include natural resource use, urbanization, alteration of stream flows through dam construction and ground-water withdrawal, modification of biotic conditions through grazing, agriculture, and introduction of non-native species, and alteration within watersheds. Better understanding of the ecology of western riparian ecosystems will increase potential for restoration and protection of remaining areas.

Establishment of woody riparian vegetation in relation to annual patterns of streamflow, Bill Williams River, Arizona

Previous studies have revealed the close coupling of components of annual streamflow hydrographs and the germination and establishment ofPopulus species. Key hydrograph components include the timing and magnitude of flood peaks, the rate of decline of the recession limb, and the magnitude of base flows. In this paper, we retrospectively examine establishment of four woody riparian species along the Bill Williams River, Arizona, USA, in the context of annual patterns of streamflow for the years 1993–1995. The four species examined were the nativePopulus fremontii, Salix gooddingii, andBaccharis salicifolia and the exoticTamarix ramosissima. We modeled locations suitable for germination of each species along eight study transects by combining historic discharge data, calculated stage-discharge relationships, and seed-dispersal timing observations. This germination model was, a highly significant predictor of seedling establishment. Where germination was predicted to occur, we compared values of several environmental variables in quadrats where we observed successful establishment with quadrats where establishment was unsuccessful. The basal area of mature woody vegetation, the maximum annual, depth to ground water, and the maximum rate of water-table decline were the variables that best discriminated between quadrats with and without seedlings. The results of this study suggest that the basic components of models that relate establishment ofPopulus spp. to annual patterns of streamflow may also be applicable to other woody riparian species. Reach-to-reach variation in stage-discharge relationships can influence model parameters, however, and should be considered if results such as ours are to be used in efforts to prescribe reservoir releases to promote establishment of native riparian vegetation.

Mapping wetlands and riparian areas using Landsat ETM+ imagery and decision-tree-based models

The location and distribution of wetlands and riparian zones influence the ecological functions present on a landscape. Accurate and easily reproducible land-cover maps enable monitoring of land-management decisions and ultimately a greater understanding of landscape ecology. Multi-season Landsat ETM+ imagery from 2001 combined with ancillary topographic and soils data were used to map wetland and riparian systems in the Gallatin Valley of Southwest Montana, USA. Classification Tree Analysis (CTA) and Stochastic Gradient Boosting (SGB) decision-tree-based classification algorithms were used to distinguish wetlands and riparian areas from the rest of the landscape. CTA creates a single classification tree using a one-step-look-ahead procedure to reduce variance. SGB uses classification errors to refine tree development and incorporates multiple tree results into a single best classification. The SGB classification (86.0% overall accuracy) was more effective than CTA (73.1% overall accuracy) at detecting a variety of wetlands and riparian zones present on this landscape.

Riparian vegetation instream flow requirements: a case study from a diverted stream in the eastern Sierra Nevada, California, USA.

A methodology is described that allows determination of instream flow requirements for maintenance of riparian trees. Tree-ring data revealed strong relationships between tree growth and stream flow volume for riparian species at Rush Creek, an alluvial stream within an arid setting; these relationships allowed development of models that predict growth rates from hydrologic variables. The models can be used to assess instream flow requirements under the assumption that certain levels of growth are necessary to maintain the population. There is a critical need for development and use of instream flow methodologies for riparian vegetation, since present methodologies focus on needs of aquatic animals (e.g., fish) and may underestimate needs of the entire riparian ecosystem.

A MANAGED FLOOD ON THE COLORADO RIVER: BACKGROUND, OBJECTIVES, DESIGN, AND IMPLEMENTATION

The Colorado River ecosystem in lower Glen Canyon and throughout Marble and Grand Canyons was greatly altered following closure of Glen Canyon Dam in 1963, as flood control and daily fluctuating releases from the dam caused large ecological changes. Ecosystem research was conducted from 1983 through 1990, and intensively from 1990 through 1995 when dam releases were modified both for scientific purposes and protection of the river ecosystem. High flows (e.g., beach/habitat building flows) were included in the Glen Canyon Dam Environmental Impact Statement (EIS), which identified a preferred strategy for dam operations and protection of the downstream ecosystem. Use of high flows partially fulfills recommendations of many river and riparian scientists for return of more natural flows, as part of initial efforts in river restoration. In 1996, a seven-day experimental controlled flood was conducted at Glen Canyon Dam to closely study the effects of a high flow event equivalent to those proposed for future da...

The importance of Olneya tesota as a nurse plant in the Sonoran Desert

. The function of Olneya tesota (ironwood) as a nurse plant and habitat modifier species in the Sonoran Desert was evaluated at five study sites (using 75 250-m2 sample plots) from Bahia Kino, Sonora to Organ Pipe Cactus National Monument, Arizona. Beneath the canopy of O. tesota trees 75 perennial plant species were found. A principal component ordination of the plots created three groups: southern, perturbed and protected sites. A strong triple association of columnar cacti, big shrubs with berry type fruits and O. tesota was detected. The relationships of Lophocereus schottii and Peniocereus striatus with O. tesota were studied in more detail. Significant differences in soil surface temperatures and stem temperatures of P. striatus were detected beneath the shade of O. tesota trees compared with soils and plants in open spaces. Olneya tesota must be considered as a habitat modifier species with ecological and conservational importance to the plant communities in the Sonoran Desert.

Change detection of wetland ecosystems using Landsat imagery and change vector analysis

Accurate, efficient, and repeatable mapping of changes in wetlands and riparian areas (referred to collectively as wetlands) is critical for monitoring human, climatic, and other effects on these important systems. We used Landsat-based satellite imagery from 1988 and 2001 to map changes in wetland ecosystems in the Gallatin Valley of southwest Montana. Stochastic gradient boosting (SGB) was used to classify the 2001 image, and change vector analysis (CVA) was used to identify locations where wetland areas might have changed between 1988 and 2001. These potentially changed locations again were classified for the 1988 Landsat image using SGB. Areas of change constituted 3.4% of the study area, thus only this small percentage of the image was reclassified for the 1988 image. Overall change detection accuracy was 76%, although changes along the periphery of wetland boundaries and in areas of smaller upland inclusions were not distinguished as well as other changes. Overall accuracies of the SGB wetland classification maps were 81% for 1988 and 86% for 2001. CVA significantly reduced the number of pixels involved in the historical image classification compared to conducting independent classifications, thus reducing the potential for compounding classification errors in unchanged areas.

Marsh development after large floods in an alluvial, arid-land river

Large expanses of riverine marsh are rare in the desert Southwest, given the dry surface of many floodplain soils. Along the Hassayampa River, riverine marsh underwent a 5-fold increase (from 2% to 9% of the floodplain-channel area) after a large winter flood in 1993. Flood waters eroded terraces that had aggraded during frequent, smaller floods, widened the channel from about 3 to 50 m, and recharged the floodplain aquifer. The net effect of these changes was a lowering of the floodplain surface relative to the water table, a variable of critical importance to riparian plant composition in arid-land rivers. Olney’s bulrush (Scirpus americanus Pers.), southern cattail (Typha domingensis Pers.), jointed rush (Juncus articulatus L.), and other obligate wetland species were abundant in 1993 and 1994 on areas with saturated surface soils or shallow water tables and often were intermixed with seedlings of early-seral tree species, including Fremont cottonwood (Populus fremontii S. Watson), Goodding willow (Salix gooddingii Ball), and salt cedar (Tamarix chinensis Loureiro and related species). The gain in riverine marsh and young cottonwood-willow stands occurred at the expense of mature cottonwood-willow forests and deep-rooted, velvet mesquite (Prosopis velutina Woot.) woodlands. Another large flood in 1995 scoured the channel of most existing vegetation and aggraded the 1993 flood channel. Early-seral tree species again established in moist soils exposed by the slowly receding flood waters. However, redevelopment of extensive marsh habitat was precluded by sediment deposition that increased the elevation of the floodplain surface relative to the water table. These changes highlight the transitory nature of riverine marsh and other vegetation patch types in the dynamic floodplains of alluvial, arid-land rivers and underscore the importance of maintaining flood flows of varying magnitude to maintain patch type diversity.

Coupling groundwater and riparian vegetation models to assess effects of reservoir releases

Although riparian areas in the arid southwestern United States are critical for maintaining species diversity, their extent and health have been declining since Euro-American settlement. The purpose of this study was to develop a methodology to evaluate the potential for riparian vegetation restoration and groundwater recharge. A numerical groundwater flow model was coupled with a conceptual riparian vegetation model to predict hydrologic conditions favorable to maintaining riparian vegetation downstream of a reservoir. A Geographic Information System (GIS) was used for this one-way coupling. Constant and seasonally varying releases from the dam were simulated using volumes anticipated to be permitted by a regional water supplier. Simulations indicated that seasonally variable releases would produce surface flow 5.4–8.5 km below the dam in a previously dry reach. Using depth to groundwater simulations from the numerical flow model with conceptual models of depths to water necessary for maintenance of riparian vegetation, the GIS analysis predicted a 5- to 6.5-fold increase in the area capable of sustaining riparian vegetation.