Source water protection (SWP) refers to the pressing global need to address the combination of issues affecting water supply and treatment: water quality, water quantity and timing of availability. Although SWP has been increasingly advocated by drinking water professionals, tools to relate upstream land-use impacts to downstream water treatability are lacking. Treatment water quality metrics can seldom be used to decouple the cumulative water quality impacts of natural and anthropogenic land-use disturbances; moreover, some changes in source water quality that potentially compromise the effectiveness of treatment are not reflected by changes in magnitude of these treatment water quality metrics. This underscores the need for an effective vulnerability indicator: a metric for monitoring and assessing the susceptibility of a system to harm arising from source water quality changes. In this research, a proof-of-concept approach was used to evaluate phosphorus as an indicator for linking source water and treatment vulnerabilities in a forested watershed in Alberta, Canada. Relationships between phosphorus and various water quality parameters were assessed using historical Elbow River discharge and Glenmore Reservoir water quality data from 2000 to 2010 to elucidate source water vulnerabilities. The results showed that allochthonous inputs from the Elbow River were the primary drivers of source water quality in the reservoir during significant hydrological events. Autochthonous processes such as dilution and deposition of allochthonous inputs in reservoir bottom sediments buffered water quality changes within the reservoir. Carbon:nitrogen:phosphorus (C:N:P) nutrient ratios observed in the reservoir were indicative of severe-to-moderate phosphorus-limitation for primary production. Although total phosphorus (TP) concentrations reached threshold levels for supporting mesotrophic to eutrophic conditions, primary production was limited. Consistently low reservoir chlorophyll-a levels and low dissolved phosphorus concentrations suggest that drinking water treatment impacts are unlikely to be confounded by the presence of algal matter. The impacts of source water quality changes to drinking water treatability were inferred using historical source water quality data and treatment performance metrics at the Glenmore Water Treatment Plant (WTP) using forward-stepwise multiple linear regression. Raw water TP and total organic carbon (TOC) concentrations explained 78.8% of the coagulant dose variation used in coagulation and flocculation processes (n = 22). Additional plant water quality data and treatment performance metrics were collected in 2012 to confirm these observations. Plant raw water UV-absorbance at 254 nm (UVA254) and TP concentrations described 98.3% of the alum coagulant dose variation used in the newly-installed Actiflo© ballasted-sand flocculation process (n = 26). SUVA and TP together explained 91.2% of the polymer dose variation (n = 26). The inclusion of TP as a significant predictor of coagulant and polymer doses, coupled with a review of literature, suggest that TP is a more sensitive indicator of waterborne particle concentrations present in the raw water than turbidity. Accordingly, TP may play a role in the critical drinking water treatment processes of coagulation, flocculation and clarification which has direct implications for process optimization, chemical coagulant costs, sludge production and impacts to subsequent treatment processes. Scenario building based on anticipated land-uses and impacts from catastrophic landscape disturbances using the coagulant regression models was used to explicitly link potential source water quality impacts to drinking water treatability. The innovative approach of estimating land-use TP loading, steady state reservoir TP concentrations and coagulant dosing requirements at the WTP provides a framework that enables the development and coordination of land-use planning, reservoir management and drinking water treatment operations strategies respectively. Total phosphorus provides a common vulnerability metric relevant to both land-use and water managers for developing and assessing integrated land-use management and SWP strategies.