A linearized, frequency-dependent analytical model is developed to examine the effect of East River on the barotropic motions in Long Island Sound. At tidal frequencies, East River creates a slightly imperfect reflecting wall at the western end of Long Island Sound, resulting in moderate reduction in the resonance of the M2 tide. At subtidal frequencies the presence of East River permits a significant amount of volume exchange through the western end of Long Island Sound, causing large scale adjustments in both the amplitude and the phase of the barotropic flow well into the interior of the Sound. It appears that the impact of East River has to be considered for a proper assessment of low frequency motion in Long Island Sound.