Interrelationship Between Watershed Condition and Health of Riparian Areas in Southwestern United States

-Sensitive hydrologic interrelationships exist between watershed condition and the health of associated riparian areas in the southwestern USA. The impact of extensive unmanaged livestock grazing, wildfires, and past forest clearing, coupled with numerous small linear perturbations such as travelways, low standard roads, and livestock trails, has dramatically illustrated the interrelationship between watershed condition and riparian health. Vegetation removal and soil compaction substantially increased surface runoff, produced sediment· laden flows, and increased erosive power to the channel system, upsetting the balance between riparian areas and the surrounding watershed. This led to the degradation, or in some cases complete destruction, ofmany riparian areas. A key factor in improving deteriorated riparian areas is understanding the balance that existed between watershed condition and riparian health in near pristine conditions. Under such conditions, watershed slopes and riparian channels were able to dissipate rainfall and concentrate flow energies produced during different precipitation events. This paper discusses the interdependency between hydrologic processes operating on upland slopes of a watershed and the channel processes affecting downstream riparian stability in the southwestern USA. A synthesis ofthis information is used to outline a method for assessing the capability of a watershed-riparian system to attain acceptable low-maintenance conditions in response to different rehabilitation treatments. Riparian areas are closely interrelated with the sur­ rounding watershed. For example, riparian communities stabilize stream channels (Riedl and Zachar 1984), provide repositories for sediment (Lowrance et a1. 1986), serve as nutrient sinks for surrounding watersheds (Lowrance et al. 1984), and improve the quality of water leaving the watershed (Schlosser and Karr 1981). They also provide temperature control through shading, reduce flood peaks by providing resistance to flow, and serve as key recharge points for renewing ground water supplies (McGlothlin et a1. 1988). However, riparian areas must be managed within the context of the entire watershed because all tributary effects cumulate to influence riparian health and stability. A delicate balance exists between riparian communities and the conditions of the watershed in which they reside. Upland watersheds in satisfactory condition absorb storm energies, provide regulation ofstormflows through the soil mantle, and, as a result, provide stability to the entire watershed. This, in tum, provides sustained flows neces­ sary for supporting healthy riparian ecosystems. In contrast, watersheds receiving past abuse have devel· oped channel systems throughout the watershed, includ­ ing ephemeral gully networks, in response to increased surface flows which cause headcutting and gully forma­ tion. These gully networks cause rapid, concentrated, sur­ face runoff which increases peak flows and produces large amounts of sediment. Past abuse and overuse of wildlands throughout the southwestern USA by grazing, trail and road construction, timber and fuel wood -harvesting, min­ ing, and other land uses have not only destroyed plant cover and increased soil erosion but also, in the process, reduced valuable riparian habitat. A large body of information is available on watershed abuse and its effect on the condition of a watershed. like­ wise, a considerable amount of information is emerging which describes the factors necessary for maintaining healthy riparian areas (e.g., streamflow duration, channel stability and configuration, grazing management, etc.). However, we are not aware of any publication which stresses the interdependency between the health of ripar­ ian areas and upstream watershed condition when devel­ oping rehabilitation strategies. Therefore, in this paper we (1) discuss the interdependency between watershed condi­ tion and riparian health, (2) identify specific hydrologic processes important for maintaining an acceptable bal­ ance between riparian areas and the surrounding watersheds, and (3) provide guidelines for developing rehabilitation strategies based on the balance between watershed condition and riparian health. Although this paper focuses mainly on rangelands in the southwestern USA, some of the general principles developed have appli­ cation on forests and brushlands throughout the western