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streams leave downstream sections virtually dry, with flowing -water riffle habitats becoming absent for up to a kilometer downstream. In some cases, however, the stream below a dewatering diversion structure receives groundwater flows due to local geologic conditions. In such cases, flowing habitats can return relatively quickly downstream, but at reduced levels compared to upstream of the diversion. Finally, some diver- sion structures are designed to bypass a near - baseflow volume of water (minimum flow) at all times. During the low flow period, reaches downstream of bypass structures appear very similar to upstream control (undiverted) reaches. We have measured the ecological effects of these three modes of operation by examining the diversity of aquatic insects (water quality indicators) and produc- tion by stream algae in downstream versus upstream reaches on several streams. Interestingly, aquatic insect indices are significantly impacted by the complete dewatering mode, and much less so by the groundwater -return or bypass -flow operations. Many of the aquatic insects that are lost below the diversion structures require flowing water, which is largely absent. The proliferation of stream algae in the dewatered reaches is also significantly higher than in upstream, naturally flowing reaches, probably due to the reduced water velocities that cannot scour away these small "plants." However, this effect was not seen in reaches with groundwater -return or bypass - flow operational modes. We did not quantify indi- ces of fish health in these streams, but we observed reduced abundances of fish in dewatered reaches and groundwater -return reaches, where stream depths were relatively shallow compared to bypass reaches. At this time, we can say that for the transbasin diver- sions in the Fraser basin, groundwater connectivity and bypass flows ameliorate some of the effects of water removal, as diverted reaches within these op- eration categories did not diverge significantly from naturally- flowing reaches with respect to some mea- sures of ecological integrity. The implications of this for restoration and management are that restoring or maintaining connectivity of surface or groundwater flows may contribute to maintenance of high ecologi- cal integrity in these systems. Threshold of impact A few previous studies in the Fraser Basin con- ducted by our group and others have indicated that when streams are diverted by less than about 50% , changes in aquatic insect communities are virtu- ally undetectable, whereas in more severely diverted reaches, significant changes in habitat and community health are observed (Rader and Belish, 1999, Pepin and Poff, 2001). Based on these observations, an- other component of our research is to examine how aquatic communities respond across a more continu- ous gradient of baseflow diversion magnitudes, from minimal to near complete. To increase the generality of our results, we are investigating diversion effects by examining this response over multiple drainage basins in the Arapahoe- Roosevelt, Medicine Bow, and Routt National Forests. With this approach we hope to discover whether there is a threshold of diversion magnitude beyond which the biological community becomes severely impaired. The identification of any Sampling Middle Fork Ranch Creek in the Fraser River basin upstream (top) and downstream (bottom) of the diversion structure. Photo credit: Julia McCarthy 2005 -�� 15