Effects of Moderate and Extreme Flow Regulation on Populus Growth along the Green and Yampa Rivers, Colorado and Utah

Abstract

River regulation induces immediate and chronic changes to floodplain ecosystems. We analysed both short-term and prolonged effects of river regulation on the growth patterns of the keystone riparian tree species Fremont cottonwood (Populus deltoides ssp. wislizenii) at three upper Colorado River Basin rivers having different magnitudes of flow regulation. We compared cottonwood basal area increment on (i) the regulated Upper Green River below Flaming Gorge Dam; (ii) the adjacent free-flowing Yampa River; and (iii) the partially regulated Lower Green River below their confluence. Our goal was to identify the hydrologic and climatic variables most influential to tree growth under different flow regimes. A dendrochronological analysis of 182 trees revealed a long-term (37 years) trend of declining growth during the postdam period on the Upper Green, but trees on the partially regulated Lower Green maintained growth rates similar to those on the reference Yampa River. Mean annual, mean growing season, and peak annual discharges were the multicollinear flow variables most correlated to growth during both pre-dam and post-dam periods at all sites. Annual precipitation was also highly correlated with tree growth, but precipitation occurring during the growing season was poorly correlated with tree growth, even under full river regulation conditions. This indicates that cottonwoods rely primarily on groundwater recharged by river flows. Our results illustrate the complex and prolonged effects of flow regulation on floodplain forests, and suggest that flow regulation designed to simulate specific aspects of flow regimes, particularly peak flows, may promote the persistence of these ecosystems.

Flow reconstructions in the Upper Missouri River Basin using riparian tree rings

“…Therefore, even without decreased annual flow, storage of spring flows for later release may decrease cottonwood ring widths. These processes may help explain the decrease in cottonwood growth rates downstream of large dams even without annual flow depletions [ Schook et al ., ].…”

“…We explored variance stabilization and prewhitening, but effects were negligible so we did not include them in the results reported. Flow reconstruction models at all three rivers were selected using the all‐subsets multiple linear regression technique with Mallow's Cp as the selection criterion [ Watson et al ., ; Schook et al ., ]. The RCS cottonwood chronologies from each site were the only three candidate independent predictors in our reconstructions (supporting information 2).…”

Flood-Rings Production Modulated by River Regulation in Eastern Boreal Canada

“…Downstream of a dam, the water table may be lowered, and the flood peaks attenuated with the frequency and magnitude of flooding reduced, suppressed, or shifted in time depending on reservoir management (Ashmore and Church, 2001;Nilsson and Berggren, 2000;Magilligan and Nislow, 2005;Graf, 2006). The composition of floodplain forests downstream of a dam often shifts to more drought-tolerant species than in the predam period with a loss of old-growth trees (DeWine and Cooper, 2007;Stallins et al, 2010;Schook et al, 2016). The response time of riparian forests varies widely across hydrological contexts.…”

Flood effects on soil thermal regimes in contrasting cold‐desert river floodplains (Yampa and Green rivers, Colorado)

“…Andersen and Nelson (2014) Cottonwood canopy cover in mature stands was~30% at DLP and 10% at BP (Figure 1). The lower value at BP reflects the current relict nature of the stand (Andersen, 2016;Schook, Carlson, Sholtes, & Cooper, 2016). Other than the presence of cottonwoods, there is currently little similarity between the Browns Park and Deerlodge Park plant communities at either intermediate or high floodplain elevations (Merritt & Cooper, 2000).…”