REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. 13, 317±334 (1997) DEVELOPMENT OF HYDROLOGICAL INDICES TO AID CUMULATIVE IMPACT ANALYSIS OF RIVERINE WETLANDS JOHN M. NESTLER AND KATHERINE S. LONG* Environmental Laboratory, US Army Corps of Engineers Waterways Experiment Station, 3909 Halls Ferry Road, Vicksburg, MS 39180±6199 USA ABSTRACT A hydrological analysis of historic stream data collected with the gauge on the Cache River at Patterson, Arkansas, USA, is presented as the basis for cumulative impact analysis of riverine wetlands. Subtle, long-term changes in hydroperiod, which could collectively have major effects on wetland function, are quanti®ed. Harmonic analysis, time-scale analysis, and conventional methods of hydrological analysis of gauge data, at decade intervals, are employed, showing a steady decline in the magnitude and predictability of the base¯ow during low ¯ow periods, beginning with the 1920s and becoming increasingly more pronounced into the 1980s. Complementary information suggests that hydroperiod alterations are associated with increased groundwater pumping to support rice agriculture in the basin. These hydrological methods are simple enough for routine application (when adequate data are available) but suf®ciently sophisticated to identify subtle changes in hydroperiod associated with cumulative effects. The changes in hydroperiod identi®ed using these methods may have potential to explain changes in biotic communities or wetlands structure as part of comprehensive wetlands studies. # 1997 by John Wiley & Sons, Ltd. Regul. Rivers: Res. Mgmt, 13: 317±334 (1997) No. of Figures: 11. No. of Tables: 3. No. of References: 33. KEY WORDS: cumulative impact assessment; environmental impact; Cache River; White River; stream¯ow; hydrology; daily values; harmonic analysis; non-linear regression models; time-scale analysis; root-mean-square error INTRODUCTION Background The importance of cumulative impacts on wetland integrity is well-documented, with the synergistic effects of several different individual impacts recognized as having collectively different effects to those of the simple sum of the individual impacts (Gosselink et al., 1990; Spaling and Smit, 1993). Single impact assessments usually focus on a speci®c activity, such as draining, ®lling, or channelization, with the speci®c nature of the impact helping to direct and focus the management of a wetland. In contrast, cumulative impact assessment attempts to describe many accumulated impacts characterizing the present state of a wetland. These impacts may be widely dispersed over many decades, occur throughout a river basin, and often lack the necessary data on present and historical land use or water use practices (Cocklin et al., 1992). Cumulative impact assessment must also address interactions of individual impacts (Spaling and Smit, 1993). The cumulative impact assessment approach presented here uses hydrological indices to describe changes in long-term discharge patterns of rivers. These hydrological indices may be linked with other information (e.g. spatial patterns in wetland vegetation) to form cause and effect sequences among wetland hydrology, spatial patterns, and functions affecting wetland habitat value. The strategy of using indexes to infer cumulative impacts in wetlands was employed by Croonquist and Brooks (1991), who used avian and mammalian guilds to indicate cumulative impacts in riparian wetlands. Developing and applying indices to describe long-term changes in CCC 0886±9375/97/040317±18 $17.50 Received January 1994 # 1997 by John Wiley & Sons, Ltd. Accepted April 1996 * Correspondence to: Katherine S. Long.