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, ebbdominated 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 hydrophysicochemical 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