The travel cost method (TCM) has been widely used to estimate values of visits to recreational and heritage sites. However, its conventional use is limited to the estimation of visitors' values given the current condition of the site. In this article, the TCM is combined with the contingent behaviour (CB) technique to allow the estimation of changes in values associated with a change in site condition. This approach provides marginal value estimates that are suited to inclusion in benefit-cost analysis. The context used to apply this conceptual development is the estimation of marginal benefits associated with improvements in the condition of the Hawkesbury-Nepean River system, an iconic waterway in NSW, Australia. Pollution, reduced flows downstream of dams and weirs, and water extraction have contributed to algal blooms and aquatic weed growth in the River. To address this decline in river health a new regime of environmental flow releases is under consideration. The increases in recreation values at six main recreation sites along the River that would result from environmental flows were estimated using the dual travel cost/CB approach. The marginal benefits were estimated to be within a range of $1.7 m to $6.6 m per annum.