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RESULTS AND DLSCUSSION Transect 1, below the area of active mining, shows little change in channel cross- section during the period 1991-1993 (Figure 2). In contrast, mining removed approximately two meu:rs of sand from the channel at Transect 2 (Figure 3). With the initiation of mining, the groundwater at Transect 2 demonstrated a rapid, sustained decline (Figure 4). Mining rapidly lowered the groundwater; initially, declines ranged from 1.45 m at the channel edge to 0.94 to at the valley wall. There was a rapid visual response of the cottonwoods to this groundwater decline including leaf-drop and branch die-back. Within two months post- mining at Transect 2, there was a decline in the crown volume of mature cottonwoods ranging from 35 at the channel edge to 5 at the valley wall. By the end of the following growing season, the groundwater declines in the mined reach ranged from approximately 2.3 m to 1.8 m. Similarly, tree mortality ranged from 15% to 75% (Figure 5). For surviving trees, crown volume decreases ranged from 15 to 45 (Figure 6) and there was little or no net increase in stem basal area (Figure 7). In contrast, change in depth to groundwater at the other transects showed smaller changes, ranging from -0.5 m at Transect 3 to +0.5 m at Transect 1. Transect 4 showed virtually no change in depth to groundwater. It is likely that mining at Transect 2 had some influence on the groundwater changes a[ Transects 1 and 3. Trenching of the channel below Transect 2 and the removal of channel fill material contributed to increased conveyance of water downstream to Transect 1. Similarly, Transect 3, located immediately upstream of the mining, appears to have been influenced by the cone of depression created by removal of channel material. Tree mortality was low at these transects during the study period. No mortality was recorded in Transect 1 and mortality in Transects 3 and 4 was <5% (Figure 5). Similarly, change in crown volume for transects 1, 3 and 4 ranged from decreases of 10 to increases of 20 (Figure 6), with Transect 3 showing the widest variability. Changes in crown volume estimates in excess of 15 appeaz to indicate change in the physiological status of the individual or groups of individuals. Finally, Transects 1,3 and 4 demonstrated positive growth over the period with net increases in stem basal area (Figure 7). CONCLUSIONS 1. The ea:cavation of sand from the channel rapidly lowered the groundwater in the adjacent riparian zone by as much as 2.3 m. 2. Sustained groundwater declines of less than 0.5 m appeared to have little effect on plains cottonwood trees. 3. Plains cottonwood was substantially affected by sustained groundwater declines of greater than 1.5 m: • decline of 15 - 45 in estimated canopy volume (scale of 0 - 100); • cessation of growth as measured by basal area increment;