Effects of Challawa and Tiga dams on the physicochemical water qualities of the below-dam sections of Challawa and Kano rivers, Nigeria

River impoundment resulting from dam construction leads to substantial alterations in the seasonal flow pattern as well as the physical and chemical qualities of the downstream water. This study assessed the effects of Challawa Gorge-dam and Tiga Dam on physicochemical parameters of surface water in the below-dam sections of Challawa and Kano Rivers, with emphasis on seasonal variations. Eight physicochemical water parameters were assessed at six stations over six months, distinguishing between reservoir and below-dam sections. In situ measurements included temperature, dissolved oxygen (DO), pH, electrical conductivity (EC), salinity, and total dissolved solids (TDS) while alkalinity and hardness were determined ex situ using titrimetric methods. Significant differences (p< 0.05), were observed between reservoir and below-dam sections of the two rivers in some parameters, the others exhibited no significant differences (p>0.05). Surface water temperature ranged from 26.90 to 30.10oC and 26.60 to 28.00oC for Challawa and Kano Rivers, respectively with significant differences (p< 0.05) during the dry and wet seasons in Challawa River. Decreasing water temperature at the below-dam sections was observed during the dry season and attributed to hypolimnetic water discharge from the reservoirs. Elevated atmospheric temperature in the dry hot season increases water loss through evaporation and consequently leads to increased concentration of EC, TD, and salinity in the reservoirs. A plausible reason for the higher concentrations of these parameters could be the non-point discharge from surrounding irrigation farms. Higher DO during the dry season was partly attributed to higher rates of photosynthetic production of oxygen as a result of elevated solar radiation, atmospheric temperature, and nutrient related parameters during the period. The variability exhibited by all the parameters in all the sections of the two rivers indicated that the parameters are within favourable ranges suitable for the survival and development of aquatic biota and fit as drinking water for man. Thus, the findings of this study considered water in all sections of the rivers fit for the development and sustainability of freshwater biota. The study concluded by recommending the need for further investigation on the effect of these dams on the distribution and abundance of different aquatic biota in the below-dam sections to guide in the adequate fisheries management measures. Key words: Impoundment, reservoir, hypolimnetic, downstream, dam, discharge, physicochemical, season