The River Continuum Concept lacks a quantitative physical model to represent downstream trends in habitat. We evaluate whether hydraulic geometry relationships can be used as a physical template to predict longitudinal trends in habitat availability and optimal flows for different life-history stages of rainbow trout (Oncorhynchus mykiss). Optimal flows based on hydraulic geometry indicate that (i) optimal flows are higher for larger fish, (ii) optimal flows proportionally increase as streams became smaller and decrease downstream, and (iii) maximum predicted habitat suitabilities for fry and juveniles are in small streams, and maximum suitabilities are displaced progressively downstream for later life-history stages. These patterns are an emergent property of downstream increases in channel depth and velocity and changes in habitat suitability curves associated with increased swimming performance of larger fish. Nonlinear downstream trends in habitat imply that fixed flow percentages recommended by the Tenant method may substantially underestimate optimal flows in small streams. Despite oversimplifying complex channel structure, hydraulic geometry relationships should serve as a useful physical model for testing downstream trends in habitat-related processes along the river continuum.

1. The pine engraver bark beetle Ips pini (Say) (Coleoptera: Scolytidae), aggregates primarily on dead or dying pine trees. In this study pine engravers were laboratory-reared on logs at a range of low densities to determine whether there was a fecundity advantage of breeding aggregations. 2. Mean reproductive success for both males and females declined exponentially with increasing density.3. Female pine engravers had shorter egg galleries at higher densities, suggesting that they left high-density breeding sites earlier. This would reduce the number of eggs that failed to survive due to larval competition.4. Some pine engravers colonized the logs voluntarily during the experiment. These volunteers settled independently of the original density.5. The fungus Ophiostoma sp. was present on the logs and may be competing with the pine engravers for limited bark area.6. Aggregation resulted in a considerable cost to pine engraver reproductive success even at low densities. Thus, it remains perplexing why pine engravers aggregate actively in nature.

Environmental gradients determine the distributions of individual species, which, in turn, shape patterns of species assemblage across those gradients. We used species distribution models to study the assemblage of fish species along the three mainstem rivers in the South Saskatchewan River Basin (SSRB) in Alberta, which flow in parallel across an 800‐km longitudinal span and down 1400 m from the Rocky Mountains to the Great Plains of North America. We estimated the similarity of species assemblages along each river to identify general patterns of species assemblage associated with temperature and five other physiochemical variables. Mean July water temperature, which ranged from <11°C at high elevation to >21°C at low elevation, was strongly associated with the presence–absence of most species in the SSRB. We found that high turnover occurred at two locations along the longitudinal gradient: where mean July water temperature was approximately 15°C and where mean July water temperature was approximately 19 or 20°C. There was also an increase in species richness at lower elevations where water temperatures were higher. Models incorporating forecasted changes in water temperature with climate change will likely provide accurate predictions of changes in the diversity and distribution of riverine fish communities across topographically heterogeneous landscapes.

Soil vapor data for benzene and four aliphatic and aromatic hydrocarbon fractions from five volatile petroleum hydrocarbon (VPH)-contaminated sites in western Canada were used together with the Canadian Council of Ministers of the Environment (CCME) Canada-Wide Standard for petroleum hydrocarbons to investigate the relative importance of benzene and the different fractions in human health risk assessment. VPH concentrations in soil vapor samples ranged from 4.0 to 4200 mg/m 3 , of which 0 to 4.6% was BTEX and 90 to 95% was hydrocarbons of the C 5-8 aliphatic fraction. VPH inhalation exposure by an adult receptor in a hypothetical, commercial building was modelled deterministically assuming 16-and 70 year occupational tenures. The magnitude of hazard quotients varied widely between sites, but their hydrocarbon fraction signatures were consistent, and characterized by higher hazard quotients in the C 5-8 and C 9-10 aliphatic and C 9-10 aromatic fractions relative to benzene and the TEX aromatic fraction. This work has shown that the C 5-8 and C 9-10 aliphatic fractions yield greater relative risk than the commonlyregulated TEX compounds, and that benzene becomes the primary chemical of potential concern only when an occupational tenure approaching 70 years is assumed.