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water surface, as the tuning of low flow often coincides with hotter drier seasons in the West. hi <br />a general sense, increased low flows provide a more reliable source of water for plants during the <br />late summer and autunul, which is beneficial to some species. In conjunction with reduced high <br />flows, increased baseflows may lead to vegetation encroachment, chamlel narrowing, and <br />reduced channel capacity. <br />Timing and rate of change in flow <br />Because some riparian species are adapted to the timing of certain flow components, altering the <br />timing inflows from historic norms may be detrimental to those species (Lytle and Poff 2004). <br />The historic timing of the peak flow on many streams in the West occurs in synchrony with the <br />timing of seed release of some riparian species (particularly the Salices: cottonwood and willow) <br />as well as the tinting of native fish reproduction. Since cottonwood trees release seed over a short <br />period of time in the spring, the seeds are dispersed at a time when newly created moist patches <br />are just being exposed in conjunction with receding water levels. <br />Delayed timing in the peak may decouple the timing of seed release from the availability of <br />suitable habitat. A delay in the peak by a few days or a week from the average date of the <br />historic peak may not cause significant reductions in cottonwood establislunent. But as the date <br />of the peak deviates further and further from the historic peak, the likelihood of successful <br />cottonwood seedling establishment diminishes. Furthermore, a delayed peak may foster <br />establishment of later dispersing species such as tamarisk, which disperse seeds from mid <br />summer through autumn in some regions in the Colorado Plateau (Cooper et ah. 1999). <br />After cottonwood seeds germnate, seedlings require ample moisture to survive. If the materiah <br />they are rooted into is coarse textured and well-drained, the water table must recede at a rate that <br />does not exceed that of the rate of root growth of cottonwood seedlings (~2.5 cm per day; <br />Mahoney and Rood 1998, Rood et al. 2005). Knowledge of when cottonwood disperse seeds and <br />the conditions under which some of the resulting seedlings are likely to survive has been <br />formalized as a conceptual model that was designed to guide managers in structuring flow <br />releases: the recruitment box model (Figure 3). The model integrates the tinting of seed release, <br />the availability of suitable habitat, and the hydrologic requirements of seedlings after they <br />germinate to help guide timing of peak flows and determining appropriate rates of stage decline. <br />This model could easily be transferred to other species and applied to other hydrograph <br />components. Site conditions of course determine how vulnerable a seedling is to desiccation, but <br />the recruitment box model provides a general approach that can be customized to a particular <br />species and a particular site to inform hydrograph design to accommodate cottonwood <br />recruitment (Figure 3). The model has been applied to prescriptive management of flows in large <br />rivers in the western US and in Canada with tremendous success in increasing cottonwood <br />recruitment (Rood et al. 2005). <br />Years following successful cottonwood seedling establishment must be sufficiently high to <br />provide moist soils for continued root growth, but can not be high enough to scour or bury <br />seedlings. With time, the seedlings become increasingly resistant to scour, less dependent upon <br />minor fluctuations in the water table, and able to sprout and adventitiously root in response to <br />burial. <br />47 <br />