Instream flows for recreation are closely correlated with mean discharge for rivers of western North America

Effective river regulation requires consideration for environmental and economic aspects and also for social aspects including recreation. Our study investigated relationships between river hydrology and recreational flows (RF) for canoes, kayaks, rafts and other non‐motorized boats, for 27 river reaches in the Red Deer and Bow river basins of southern Alberta, Canada. A subjective RF method involved regression analyses of data from River Trip Report Cards, volunteer postcard‐style surveys rating flow sufficiency. A total of 958 trip reports were submitted for the rivers between 1983 and 1997 and about 30 reports permitted confident regression analysis for a river reach. Values from these analyses were very consistent with values from the ‘depth discharge method’, a hydraulic modelling approach that used stage–discharge ratings to determine flows that would produce typical depths of 60 and 75 cm for minimal and preferred flows, respectively. Values were also consistent with expert opinions from river guidebooks and maps and aggregate values were calculated from the combined RF methods. These were very closely correlated with mean discharge (Q_m) across the rivers (r² = 0.94 for minimal and 0.96 for preferred flows). The relationship best fitted a power function (straight plot on log versus log scales) with a consistent slope but vertical offset for minimal versus preferred flows. Close relationships between guidebook estimates of RF and Q_m were also observed for rivers in the American Rocky Mountain states of Idaho (r² = 0.55 and 0.74), Montana (r² = 0.34 and 0.80) and Colorado (r² = 0.43 and 0.51), but the association was weaker for the Pacific Northwest state of Oregon (r² = 0.35 and 0.26). These analyses indicate that RF can be confidently determined through a combination of subjective and hydraulic methods and reveal that RF values represent a systematic function of discharge for a broad range of alluvial and constrained river reaches. From these analyses we provide the ‘Alberta equation’: minimal recreational flow = 3 × Q_m^0.59 (Q_m in m³/s), and preferred flows would typically be 1.5 times higher. For other river regions the exponent ‘0.59’ may be relatively constant but adjustments to the coefficient ‘3’ could be applicable. Copyright © 2005 John Wiley & Sons, Ltd.