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populations at Bishop ring width (RW,-1!. 'I:) or during May 2 df F p 's 19 20.4 <0.001 16 19 40.6 <0.001 '4 19 28.7 <0.001 :0 19 39.0 <0.001 vely). Radial growth low reach was high vigor was low and lely abundant. o 0 o OREACH ~ . REACH 4 6 7 e 9 10 11 ~ ~ ~~~ 6. REACH 2 . , . (mm) 7T class (on a scale of lation toaverilge tree 8 for reaches 2, 4, and ~ssion equations are: {' = 0.31, P < 0.002, . 0.05x', R' = 0.64, P ~ 0.8x - 0.05x', R' = January 1991 I 030083 DISCUSSION Diverted Reaches Growth of Fremont cottonwood and black cottonwood (obligate riparian trees) has been limited by water availability in many years in the three partially diverted reaches of Bishop Creek examined in this study. Annual ring widths of trees in the diverted reaches have averaged between 2 and 4 mm. and canopy vigor of the trees is low. Instream flows for maintaining healthy vigorous populations of cotton- wood can be estimated based on the rela- tionships described in this study between: (I) annual growing season flow and annual ring width, (2) ring width and canopy vig- or. and (3) ring width and population mor- tality. Results for the latter two relation- ships indicate that healthy populations of cottonwood maintain average annual ring width of at least 3-4 mm depending on ~levation. The flows that will increase growth to such levels can be estimated us- ing instream models. For example, to in- crease growth of reach 4 black cottonwood from 1.9 mm (= 1 on the standardized ring- width scale for the reach) to 3 mm (stan- dardized ring width of 1.6), simulations indicate that annual flows must be ca. 35 hmJ. Similar calculations for the black and Fremont cottonwoods in reach 4 yield flow values of 59 and 48 hm3. respectively. Another approach to determining accept- able flow volumes for maintenance of cot- tonwoods is to use ring-width values at- tained by non water-stressed trees such as those in the supplemented-flow reach as growth "standards." Such an approach yields similar results, because the average growth rate of these trees (4.2 mm) is near- ly equal to the minimum rate empirically associated with maintenance of healthy canopies. Flows that are estimated to maintain healthy cottonwood stands at Bishop Creek (ca. 35-S9 hm3/year) are about three to four times higher than average flows in the di- verted reaches over the last 20 years (ca. 13.5 hmJ/year). almost half as much as that released below power plant 6 (104 hm3/ year). and about 40-60% of the estimated natural flow (93 hm3/year) in the stream. Results from another diverted alluvial eastern Sierra Nevada stream, Rush Creek, also suggest that a large percentage of the natural flow is needed for maintenance of cottonwood popuLations (Stromberg and Pallen 1990). The flow-growth models for the two streams (Bishop and Rush creeks) explain a similar amount of the variation in annual growth (75-90%), with stream- flow explaining the largest fraction of the total variance for both (Stromberg and Pat- ten 1990). This is significant in that it in- dicates a robust trend for dependency of cottonwood growth on flow volume with- in eastern Sierra Nevada streams. It also suggests that generalizations can be made about growth relations and flow volume requirements for riparian trees in eastern Sierra Nevada streams, a group considered to respond individualistically to diversion (Harris et al. 1987). The mechanism underlying the in- stream flow models in the diverted reaches is the positive growth response to in- creased water availability in riparian soils during high-flow years. Growth of trees. however, was also influenced by a com- pounding factor of flood flows, as evi- denced by the disparate patterns of post- 1982 growth among trees. The flood flows of 1982 probably caused structural damage to some riparian trees (e.g., cambial dam- age or breakage of limbs or roots) or to their habitat (e.g., scouring of the rhizo- sphere substrate) that negated positive ef- fects of increased water availability. The trees that showed growth reduction in 1982 were those growing within 1 or 2 m of the stream where substrates are most suscep- tible to erosion. The restriction of many cottonwoods at Bishop Creek to a narrow strip (<5 m) along the streambed may be a result of diverted streamflow, and it pre- disposes the trees to greater impact from flood flows. Although dams and diversions may reduce the frequency of low-magni- tude flood flows and allow encroachment of trees into the channel (Harris et al. 1987), they do not eliminate the infrequent large- magnitude flood flows and may ultimately increase the damage to riparian trees. Influent versus Effluent Reaches Trees in the influent and effluent di- verted reaches generally had similar I j. C. Stromberg and D. T. Patten 9 I I~