Temperature and salinity data from Long Island Sound (LIS) show that the pycnocline rises seaward of Mattituck sill and that the magnitude of the longitudinal density gradient over the area of the sill remains relatively constant throughout the year. This reflects the influence of internal hydraulic processes and vertical mixing on hydrography and volume exchange. Records of moored instruments indicate intratidal and fortnightly variations of water column stratification and current velocity in eastern LIS. During spring tides, thick bottom mixed layers reveal vertically homogeneous temporal changes in temperature, salinity, and sigma-t, which arise from the oscillatory advection of a longitudinal density gradient. Intratidal variations translate into barotropic rectification of the residual flow and vertical homogeneity of low-passed temperature, salinity, and sigma-t. During neap tides the flow may be subject to hydraulic control. Near-surface temperature and salinity oscillations of higher amplitude than near-bottom fluctuations are a reflection of vertical excursions of the halocline. The halocline is raised in flood periods and depressed in ebb periods. Intratidal stratification changes reflect the corresponding low-passed temperature, salinity, and sigma-t behavior with depth. Near-bottom residual flow favors salt intrusion as it accelerates down the salinity gradient due to the adjustment of the longitudinal density gradient established on spring tides. The residual density and flow fields reflect a change in dynamical balances from spring periods, dominated by frictional stresses, to neap periods, when inertial and rotation effects increase. ¿ American Geophysical Union 1994