To facilitate extrapolation among watersheds, ecological risk assessments s
hould be based on a model of underlying Factors influencing watershed respo
nse, particularly vulnerability. We propose a conceptual model of landscape
vulnerability to serve as a basis for watershed classification systems to
predict resistance and resilience of aquatic ecosystems to hydrology-relate
d stressors. Watershed area, storage capacity, channel slope, and soil perm
eability determine sensitivity of lotic systems to stressors associated wit
h land-use activities that impact hydrologic regimes. Natural hydrologic di
sturbance regimes also influence the resilience of aquatic systems by selec
ting for life history strategies associated with rapid recolonization follo
wing disturbance. Variability in some of these physiographic driving factor
s can be partitioned by landscape classification schemes such as the U.S. F
orest Service Ecological Unit Classification System, while others (watershe
d storage) may explain remaining variability within landscape units. We are
conducting a comparative watershed study to examine simple and interactive
effects of physiographic units, watershed storage (lakes + wetlands), and
land-clearing activities in watersheds surrounding the western arm of Lake
Superior. Initial results for second-order watersheds indicate significant
watershed class effects on baseflow water quality, percent motile biraphid
diatom species in periphyton communities, habitat quality, and fish communi
ty integrity. Future studies have been designed to examine cumulative effec
ts downstream.