The relationship between artificial tidal restrictions and shellfish-water quality was studied within otherwise sparsely developed estuaries on Cape Cod (Massachusetts, USA). The primary study site, the 600-ha diked Herring River (Wellfleet, MA) estuary, has had a long history of shellfish-water closures because of water-column fecal-coliform (FC) contamination despite the lack of human fecal sources. Ongoing efforts to restore tidal flow to the system have raised questions about the effects on microbiological water-quality on extensive beds of wild oysters (Crassostrea virginica) and cultured hard clams (Mercenaria mercenaria) in the river mouth. This study combines observations of current spatial and temporal (tidal) patterns of water-column contamination with recent hydrodynamic modeling to predict the effects of proposed tidal restoration on shellfish-water quality. Under presently tide-restricted conditions, high FC is restricted to about 1000 m on either side of the dike structure and only during low tide, preventing the harvest of extensive natural oyster beds; farther downstream, hard-clam aquaculture is marginally protected by relatively coliform-free, high-salinity Cape Cod Bay waters. Modeling of Herring River under tide-restored conditions showed that a predicted 13-fold increase in river intertidal volume, over existing tide-restricted conditions, would dilute measured FC to concentrations that are acceptable for shellfish-growing waters. Restored tidal flow would also reduce coliform survival time by increasing salinity, dissolved oxygen and pH, all presently depressed throughout the system because of the biogeochemical disturbance of diking and drainage. Results from Herring River, plus a preliminary survey of other diked Cape Cod estuaries, suggest a direct relationship between the degree of tidal restriction and surface-water FC, which should be studied further.