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The primary objective of the Northern Rivers Basins Study (NRBS)was to advance the understanding of how anthropogenic developments haveimpacted the ecology of the Peace, Athabasca and Slave rivers, Canada.To investigate these multiple stressor impacts, the NRBS used acumulative effects assessment (CEA) based upon a weight of evidenceapproach. This CEA determined the exposure and effects of multiplestressors on these systems and included experimental verification ofcausality and linkage of this understanding to field data. Fish studiesconfirmed that dioxins and furans occur in the food web at low levelsacross the basins; detection frequencies for all2,3,7,8-substituted dioxin/furan congeners (2,3,7,8 TCDD/TCDF)were highest near bleached kraft pulp mills. PCDD/Fs, rather thanPCBs or organochlorine pesticides, were the chemicals of concern forhuman exposure. In addition, the recent decline in concentrations ofdioxins and furans in fish was commensurate with improvements in pulpmill treatment technologies. Throughout the basins, enrichment occurredbelow pulp mill and municipal discharges. Multiple stressor effects ofpulp mill effluent were separated from one another by using experimentsthat demonstrated that the primary impact of the mill effluent wasnutrient enrichment. River state was also greatly influenced byhydrologic impacts. In particularly, the Bennett Dam changed the ecologyof the Peace-Slave river-delta systems more than 1500~km downstream,while climate variability was an important contributing factor for thesedelta impacts. Notable contributions of the NRBS model were thecooperation among governments, aboriginal peoples, non-governmentorganizations, industry and other stakeholders, and the two-waycommunications flow among the scientific community and these groups.NRBS science was also deemed relevant because 16 questions forwarded bystakeholders focused the Study's CEA on specific issues of importancewithin the basin. In summary, the NRBS model of CEA included four keycomponents: (1) communication of stakeholder priorities to producerelevant science questions; (2) pursuit of the best availablescientific understanding of ecosystem impacts by engaginginterdisciplinary science teams; (3) assessment of futureenvironmental condition through monitoring and follow-upinvestigation; and (4) communication of results to managers toproduce appropriate regulatory change. This CEA model combines theseelements with scientific rigor to generate meaningful information tostakeholders while also improving regional assessments of riverbasins. [ABSTRACT FROM AUTHOR]