studies in Pelorus Sound. These show the importance of the large, variable freshwater input into Pelorus Sound. The Sound has 380 kilometres (240 mi) of shoreline and an area of . Average freshwater input is , of which the Pelorus and Kaituna Rivers at the head of the sound deliver respectively. Rain falling directly on the surface of the sound provides another , and the remainder drains from adjacent land via small streams. This freshwater input corresponds to a depth of of water distributed over the area of the sound per year. The freshwater input is highly variable. In a typical year there are several events where the daily rainfall exceeds and the river flow exceeds . The freshwater inflow generates an estuarine circulation in the main channel, with low-salinity water moving outwards at the surface, and higher-salinity water flowing inwards below. The currents in the main channel associated with this circulation are typically , but immediately after a flood event they can be much larger. Fresh water layers have been observed to spread through the Sound at . After each flood event, the freshwater is slowly flushed out of the sound, with a flushing time estimated to be 21 days. The tidal regime at the entrance to Pelorus Sound is typical of New Zealand coastal locations, i.e. there is a semi-diurnal tide modulated on a spring-neap cycle, generated by the superposition of M2 (lunar, semi-diurnal) and S2 (solar, semi-diurnal) tidal constituents. Other tidal constituents, notably N2 (semi-diurnal, generated by the ellipticity of the moon's orbit), introduce further modulation, meaning that the spring-neap cycle is not exactly repeated. The tidal range varies between at neap and at spring. (These values are calculated from a sea level time series, July 1997 – February 1998, at Pelorus Sound entrance.) The propagation of this tidal signal into Pelorus Sound has not been well described, but models and measurements both suggest that the tidal range within the sound is somewhat larger (by 10–20%) than at the entrance. Because the sound is long, and in places narrow, the tidal currents are fairly strong. A further factor affecting the hydrodynamics of Pelorus Sound is wind. Winds in the area are generally strong and are channelled by the steep topography. Wind stress acting on the surface generates currents, which have not been well described. The wind stress also generates turbulence, which causes the surface fresh water to