The influence of river regulation and land use on floodplain forest regeneration in the semi‐arid upper Colorado River Basin, USA
ABSTRACT Flow regulation effects on floodplain forests in the semi-arid western United States are moderately well understood, whereas effects associated with changes in floodplain land use are poorly documented. We mapped land cover patterns from recent aerial photos and applied a classification scheme to mainstem alluvial floodplains in 10 subjectively selected 4th order hydrologic units (subbasins) in the Upper Colorado River Basin (UCRB) in order to document land use patterns (floodplain development) and assess their effects on Fremont cottonwood forest (CF) regeneration. Three of the mainstem rivers were unregulated, five were moderately regulated and two were highly regulated. We classified polygons as Undeveloped (with two categories, including CF) and Developed (with five categories). We ground-truthed 501 randomly selected polygons (4–28% of the floodplain area in each subbasin) to verify classification accuracy and to search for cottonwood regeneration, defined as stands established since regulation began or 1950, whichever is most recent. From 40% to 95% of the floodplain area remained undeveloped, but only 19–70% of the floodplain area was classified as forest. Regeneration occupied a mean of 5% (range 1–17%) of the floodplain. The likelihood of the presence of regeneration in a polygon was reduced 65% by development and independently in a complex manner by flow regulation. Our analyses indicate that floodplain forests may be in jeopardy on both regulated and unregulated rivers and that information on historical forest extent is needed to better understand their current status in the UCRB. Conservation efforts need to be coordinated at a regional level and address the potentially adverse affects of both flow regulation and floodplain development. Published in 2007 by John Wiley & Sons, Ltd.
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ABSTRACT: In arid regions of the world, the conversion of native vegetation to agriculture requires the construction of an irrigation infrastructure that can include networks of ditches, reservoirs, flood control modifications, and supplemental groundwater pumping. The infrastructure required for agricultural development has cumulative and indirect effects, which alter native plant communities, in parallel with the direct effects of land use conversion to irrigated crops. Our study quantified historical land cover change over a 150-year period for the Walker River Basin of Nevada and California by comparing direct and indirect impacts of irrigated agriculture at the scale of a 10,217 km2 watershed. We used General Land Office survey notes to reconstruct land cover at the time of settlement (1860–1910) and compared the settlement-era distribution of land cover to the current distribution. Direct conversion of natural vegetation to agricultural land uses accounted for 59 percent of total land cover change. Changes among nonagricultural vegetation included shifts from more mesic types to more xeric types and shifts from herbaceous wet meadow vegetation to woody phreatophytes, suggesting a progressive xerification. The area of meadow and wetland has experienced the most dramatic decline, with a loss of 95 percent of its former area. Our results also show Fremont cottonwood, a key riparian tree species in this region, is an order of magnitude more widely distributed within the watershed today than at the time of settlement. In contrast, areas that had riparian gallery forest at the time of settlement have seen a decline in the size and number of forest patches. La conversión de vegetación nativa en agricultura en las regiones áridas del mundo demanda la construcción de una infraestructura de irrigación que incluya redes de canales, reservorios, obras para el control de inundaciones y bombas para extraer agua subterránea suplementaria. La infraestructura necesaria para el desarrollo agrícola en esas regiones tiene efectos indirectos y acumulativos, que afectan las comunidades de plantas nativas, paralelamente con los efectos directos de la conversión del uso del suelo a cultivos irrigados. En nuestro estudio se hizo la cuantificación del cambio histórico de la cubierta del suelo sobre un período de 150 años en la Cuenca del Río Walker de Nevada y California, comparando los impactos directos e indirectos de la agricultura de irrigación en un ámbito de 10.217 km2 de cuenca. Utilizamos las notas de los estudios realizados por la Oficina General de Tierras para reconstruir la cobertura del suelo en los tiempos del poblamiento del área (1860–1910) y comparamos la distribución de la cubierta del suelo durante la era del poblamiento con la distribución actual. La conversión directa de la vegetación natural en usos agrícolas representó el 59 por ciento del cambio total de la cobertura de la tierra. Los cambios experimentados en la vegetación no agrícola incluyeron transformaciones desde tipos más mésicos a otros más xéricos, lo mismo que cambios de vegetación de pradera herbácea húmeda a freatofitos leñosos, lo cual indica una xerificación progresiva. El área de praderas y humedales ha experimentado la declinación más dramática, con una pérdida del 95 por ciento de su extensión anterior. Nuestros resultados también muestran que el álamo de Fremont, una especie arbórea ribereña clave de la región, se encuentra hoy en un orden de magnitud más ampliamente distribuida dentro de la cuenca de lo que estuvo en tiempos del poblamiento. Por contraste, las áreas que tenían bosque de galería ribereño en tiempos del poblamiento han visto una declinación en el tamaño y número de los parches de arbolado.Annals of the Association of American Geographers 05/2012; 102(3):531-548. · 2.09 Impact Factor
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ABSTRACT: Summary1. Hydropower is often presented as a clean and renewable energy source that is environmentally preferable to fossil fuels or nuclear power. Hydropower production, however, fundamentally transforms rivers and their ecosystems by fragmenting channels and altering river flows. These changes reduce flow velocity and the number of rapids, and reduce or alter wetland, floodplain and delta ecosystems. Dams disrupt dispersal of riverine organisms and sediment dynamics and may alter riverine biodiversity composition and abundance. Freshwater ecosystems now belong among the world’s most threatened ecosystems.2. Water managers are beginning to recognise the need to combine demands for social and economic development with the protection of the resource base on which socio-economic benefits rely. Environmental flows can help to balance ecosystem and human needs for water, both when constructing new dams and in re-licensing existing dams.3. We briefly review the impacts of hydropower generation on freshwater ecosystems by discussing different types of dams and development, and by providing examples from Sweden of how environmental effects of hydropower production could be mitigated. Special emphasis is given to flow regulation through re-operation of dams.4. Regulated rivers in Sweden were developed with little consideration of ecological effects, with most dams lacking migration pathways or minimum flow releases. There is thus a substantial potential for improvement of ecological conditions, such as naturalisation of flow regimes and reestablishment of connectivity, in regulated river reaches but technical hurdles imply major challenges for rehabilitation and mitigation. Most regulated rivers consist of cascades of consecutive reservoirs and impoundments, further constraining possible actions to improve ecological conditions.5. Most environmental mitigation measures require flow modifications to serve ecosystems, implying reduced power production. An important challenge for river management is to identify situations where measures involving relatively small production losses can have major ecological advantages.6. Climate change during the 21st century is expected to increase runoff in northern and central Sweden and make the annual hydrograph more similar to variation in electricity demand, i.e. a lower spring flood and increased run-off during winter months. This could provide opportunities for operating dams and power stations to the benefit of riverine ecosystems. On the other hand, demands to produce hydropower are likely to increase as fossil fuels are phased out, leading to increased pressures on free-flowing rivers and aquatic ecosystems.Freshwater Biology 12/2009; 55(1):49 - 67. · 3.93 Impact Factor
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ABSTRACT: Hydrological and land use changes can affect species in human altered landscapes. Typically the impacts of hydrological and land use changes are examined separately, with hydrological determinants used to explain the distribution of species in water dependent and aquatic habitats and land use factors used to examine terrestrial species. However, given the connectedness of aquatic and terrestrial habitats, stressors originating in one domain may be important in the other. To explore the importance of integrating both hydrological and land use factors, we tested the relative contribution of hydrological factors and land use context as determinants of the dominant riparian tree species, Eucalyptus camaldulensis Dehn. throughout wetlands of the Condamine catchment, southern Queensland, Australia. The occurrence of E. camaldulensis was modelled against hydrological and land use factors using generalized linear models (GLMs) and validated using internal bootstrapping procedures. Validated models which included both hydrological (distance from weir, wetland–river connectivity and groundwater depth) and land use factors (agricultural land cover and grazing intensity) performed better than those developed using only hydrological factors. The study results highlight the importance of an integrated perspective which considers both hydrological and land use factors in order to understand occurrence patterns of riparian and floodplain tree species in a range of settings. This approach could be especially important when assessing changes to hydrology and land use which may be triggered by climatic changes.Austral Ecology 03/2014; · 1.74 Impact Factor