The hydrology of the 6000 sq km basins of the rivers Tay and Earn is examined from the special point of view as the source of freshwater for the middle and lower parts of the Tay estuary. Discharge frequency curves and average inflow figures are presented and analysed. Using these flow figures and chemical analyses obtained since the completion of the Hydro-Electric schemes estimates of the dissolved oxygen, Biological Oxygen Demand and suspended solids load transported into the estuary have been prepared.

An understanding of the movement of a body of water can be considered a prerequisite to any other study whether it be physical or biological. This is particularly so in the case of an estuary where the water movements are complex. The movement of water in an estuary is primarily governed by the tidal and freshwater inputs, the resulting flow being modified by bed and channel configurations, salinity, and siltload variations. The tidal and freshwater inputs are themselves variables, the tidal range varying with the lunar cycle, and the freshwater input varying with the season or rainfall. The salinity variations within an estuary are interlinked with the tidal and freshwater fluctuating inputs. In addition the local and regional weather can modify the tidal response.

Salinity distribution in the Tay estuary measured during the years 1969–70 is influenced by freshwater flow and tidal range. Between Pool and Flisk, tidal-depth-mean salinity is a strong function of river flow but is much less influenced by tidal range. The relationship between salinity range (difference between maximum and minimum depth-mean salinities) and tidal range can be approximated by a linear function. The observed vertical salinity distribution varies with tidal state. The net non-tidal circulation has been estimated at Pool, Broughty Ferry, Newport, Balmerino and Flisk, and found to be dependent on river flow and position.

A lack of modern river-borne sediment and a steady supply of marine materials have led to the hypothesis that the upper Tay estuary is undergoing an erosive phase counterbalanced by a headward moving sheet of largely marine-derived sand presently occupying the upper-middle estuary. This primal division (based on sediment budget criteria) is also reflected in the bathymetry, distribution of sediment types, and sedimentary structures. The situation of two contrasting ‘phase zones’ juxtaposed within a relatively small area is ideal for an empirical and indirect analysis of the response of a wide variety of sediment types to the gross physical processes acting on them. The analysis takes the form of standard grain-size characterisations using conventional sedimentary statistics, supplemented by adaptations of C-M and QDa-Md diagrams transformed for display on distribution maps.

During periods of sustained moderate river discharge and quiescent marine conditions little external suspended sediment enters the estuarine circulation of the Tay. That material which is in suspension is largely derived from the estuary margins where tidal currents superimposed by wind-induced waves are competent to resuspend fine material from the surface of the ‘mud’ flats and erode bedded silts from the incised banks of minor channels and runnels draining them. The quantities of this sediment entering the system are largely determined by tidal state and amplitude, as well as wind velocity.

On spring tides the flats are entirely covered at high water, and dry out completely at low water. The volume of water and its areal coverage at high tide ensures that, during the ebb, water charged with high concentrations of suspended sediment is directed from the fiats into the surface and middepth waters of the main channel. This process acting along the 20 km length of the channel flanking the ‘mud’ flats, combined with the low tide ‘ponding effect’ caused by the tide flooding from the sea while the upper estuarine water is still ebbing, results in the cumulative formation of a zone of high suspended sediment concentrations (turbidity maximum). As the flood tide becomes fully established the zone is diluted and dispersed. During neap tides the same processes operate, but because a smaller area of the flats is covered at high water and uncovered at low water, and because neap tidal current speeds are lower than those for spring tides, two proportionally weaker zones are recognised.

Following periods of sustained moderate river discharge, quiet sea conditions and calm weather, suspended sediment concentrations in the Tay are negligible irrespective of tidal state or amplitude.

The southern entrance shores to the Tay estuary form an area of extensive inter-tidal sedimentation. Tidal currents of varying strengths characterise the area and the situation is modified by both wind and wave. The net result is a series of seven sub-environments of sedimentation. The textural characteristics of the sediment and the distribution of major and minor sedimentary structures in each subenvironment is closely linked to stability and changing physical processes. The distribution of organisms, live and dead, provides a good indication of these patterns of sediment and stability.

The distributions of benthic macroflora and macrofauna in the Tay estuary were investigated qualitatively and quantitatively during the spring and summer months of 1973 and the results compared with the study by Alexander et al. (1935). Sublittoral areas were sampled using SCUBA diving techniques. The overall distribution of flora and fauna is determined primarily by salinity variation and substrate texture, a large part of the sublittoral area being found to have little or no infaunal component due to the instability of the coarse sediments found widely in this region. The most prominent sublittoral populations are those of Mytilus edulis found in the middle and outer estuary. The northern shore of the inner estuary is composed entirely of mud and sand flats bounded by large areas of Phragmites communis. The predominant organisms in this area are euryhaline annelid and crustacean species. The southern shore is comparatively narrow and is composed of a variety of substrates from bedrock to mud. The flora and fauna of this shore varies with substrate type but it is here that macroalgal species are found to penetrate the inner estuary. Some euryhaline organisms penetrate the estuary as far as Newburgh, the point where freshwater and estuarine populations overlap. No evidence of gross pollution was detectable in this study except in the immediate vicinity of sewage outfalls.

Two features of the effects of salinity on the bacterial flora of the Tay estuary are considered. First the effect of salinity on the size and species composition of the resident microfiora at various sites within the estuary. The second part is concerned with data obtained from laboratory experiments designed to study the effects of subjecting marine and freshwater bacteria, isolated from the Tay, to salt stress.