Estuaries of southwest England: Salinity, suspended particulate matter, loss-on-ignition and morphology

Analyses of consistently measured survey data and published hydrological data for the estuaries of southwest England show that: (1) freshwater runoff and estuary length are strongly related to catchment area; (2) salt intrusion length is always greater than 74% of the estuary length during low runoff, spring-tide summer conditions and, on average, is more than 90% – there is a statistically significant relationship between salt intrusion length and tidal range (increased intrusion) and freshwater runoff (decreased intrusion); (3) salinity stratification is strongly, positively correlated with the Simmons’ Ratio – a measure of the relative balance of freshwater- and tidally-induced current speeds; (4) residence times are strongly correlated with tidal range and tidal length and somewhat less strongly correlated with tidal prism – longer estuaries with smaller tides have longer residence times than shorter estuaries with stronger tides; (5) maximum surface suspended particulate matter (SPM) concentrations vary from 2 to 20,000 mg l −1 ; (6) maximum surface SPM concentrations are strongly related both to tidal range and tidal length and less strongly correlated with tidal prism – long, strongly tidal estuaries have greater concentrations; (7) loss-on-ignition (LOI) of SPM is strongly related to SPM concentration – low concentrations (less than a few mg l −1 ) typically have LOI > 50% and high concentrations (greater than a few hundred mg l −1 ) typically have LOI ∼ 10%; (8) LOI of the freshwater SPM is greater than that in the area of maximum estuarine SPM concentration, so that SPM within the high-turbidity reaches of southwest estuaries is significantly different from that entering the estuaries in their freshwater inflows; (9) an observed scaling relationship between estuary tidal length and a prescribed function both of depth at the mouth and tidal range is similar to that given by theoretical work (Prandle, 2004; Prandle et al., 2006), as is an observed scaling relationship between depth at the mouth and mean annual runoff. To our knowledge, the measurements and results presented here for the estuaries of southwest England are, in many cases, the first that quantify their important in-water physical characteristics and behaviour. Apart from presenting new information, the synthesis has some important implications for estuarine management, and may aid the straightforward estimation of some key physical variables for these systems.