Philadelphias water resources are widely used for a range of recreational activities and since almost 60% of the City is served by Combined Sewer Systems (CSS), the pathogen levels after wet weather discharges are of main concern for evaluating the hygienic status of Citys these resources. A quantitative microbial risk assessment (QMRA) was conducted to estimate risk of gastrointestinal illnesses (GI) associated with recreational exposures to Philadelphia water bodies, under dry and wet weather conditions. Usage pattern was measured at ten different locations (creek sites: n=3 and river sites: n=7) by using a novel ``time lapse photography technology during three recreational seasons (May-September) 2008-2010. Using maximum likelihood estimation stochastic exposure models were generated for each exposure scenario. In general, the log normal distribution described the playing and wading duration distribution, while the gamma distribution was the best fit for fishing durations. Both E. coli and Enterococcus were evaluated as water quality indicators and daily risk of GI illnesses in proportion of the frequency of use, including all types of activities that span the range of exposure at each site, was predicted by running 10,000 Monte Carlo simulations in Crystal Ball. An exponential dose response model was developed based on the 1986 US EPA epidemiological studies and the results were compared with the EPAs 1986-criteria model. A base line approach was developed to extend the risk estimates from local-observation stations to the entire stretch of the water bodies and critical zones of the Schuylkill and the Delaware River were identified for further investigations. Consistently, the sites on small creeks presented elevated risk potential under episodic events of poor water quality. Activities resulting in greatest number of affected users at creeks were identified as wading and playing (81%), while fishing was the potential risk contributor (65%) at rivers. To our knowledge this is the first QMRA to employ time-lapse cameras to characterize exposure types and durations. The quantitative measure of risk contribution from each type of water activity and identification of critical zones on large water bodies can be useful for policy makers in planning the health campaigns and prioritizing the future interventions.