Physicochemical Characteristics of the Great Kwa River (Gulf of Guinea): Controls of Tidal Dynamics and Environmental Sustainability in a Tropical Mesotidal System

Abstract

The Great Kwa River, a tidal tributary of the Cross River Estuary in the Gulf of Guinea, was investigated to evaluate the influence of tidal dynamics on physicochemical characteristics and environmental sustainability in a tropical mesotidal system. A total of 50 bottom-water samples were collected along a 17.65 km tidal reach during the rainy season and analyzed in situ for temperature, pH, electrical conductivity (EC), salinity, turbidity, transparency, and total dissolved solids (TDS), while tidal current velocity and bathymetry were simultaneously measured. The river exhibited a semi-diurnal, ebb-dominated tidal regime, with maximum ebb current velocities of 81–84 cm s⁻¹, exceeding flood velocities (68–76 cm s⁻¹), indicating strong fluvial–tidal interaction. Physicochemical parameters remained within freshwater limits, with temperature ranging from 25 to 28 °C (mean 26.4 ± 1.03 °C), pH from 6.0 to 7.5 (mean 6.9 ± 0.4), EC from 22 to 58 µS cm⁻¹ (mean 33.5 ± 8.4 µS cm⁻¹), salinity from 0.014 to 0.037‰ (mean 0.021 ± 0.005‰), transparency from 26 to 106 cm (mean 68.3 ± 16.6 cm), turbidity from 1.51 to 12.71 NTU (mean 5.31 ± 2.36 NTU), and TDS from 12.1 to 31.9 ppm (mean 18.4 ± 4.6 ppm). A distinct downstream increase in salinity, EC, and TDS, accompanied by decreasing temperature and pH, reflected progressive tidal influence and limited estuarine mixing. Three hydro-physicochemical facies—upper fluvial, middle transitional, and lower tidally influenced—were identified, representing successive zones of freshwater transport, active sediment reworking, and fine-sediment deposition. Strong positive correlations among salinity, EC, and TDS confirmed ionic concentration as the principal control on water chemistry. Although the river presently maintains good freshwater quality, localized turbidity maxima and increasing downstream ionic concentrations indicate emerging environmental sensitivity. These findings provide an important baseline for long-term monitoring and support sustainable management of the Great Kwa River and comparable tropical tidal rivers within the Gulf of Guinea.