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River impoundment is usually accompanied with consequences of alteration in the hydrological system and change in the biodiversity composition of the ecosystem. This study investigated the impact of the dams built on Challawa (that is, Challawa George-Dam) and Kano (that is, Tiga Dam) on the fish biodiversity of the reservoir and below-dam sections of the two rivers. Fish sampling was done using long lines, Malian traps, cast and gill nets from stations on the reservoir and below-dam sections of the rivers. Ten fish species were identified from the reservoir and the below-dam sections of Challawa River belonging to nine families and four orders. Fish communities of the reservoir and below-dam sections of Kano River consisted of 28 species belonging to 12 families and 7 orders. Results of the Shannon diversity index (H' ) indicated that the two reservoirs have relatively higher fish species diversity measures (that is, Challawa=1.948 & Kano=2.294) than the below-dam parts (that is, Challawa=1.833 & Kano=2.247) of their respective rivers. Non-metric Multidimensional Scaling (NMDS) analysis produced two-dimensional maps with stress values of 0.0554 and 0.0537, an indication that the model provided good representation of the original data in reduced-dimensional space. The ordination map indicated wide dissimilarities between fish communities of the impounded and below-dam sections. Furthermore, a one-way analysis of similarity indicated significant difference (P=0.0001) and high dissimilarities (R=0.643) between fish communities in the two sections of Challawa River and also in Kano River (P=0.001, R=0.929). Analysis of Similarity Percentage indicated 74.45% of the dissmilarities between fish communities in the two sections were cummulatively contributed by four species in Challawa River and 72.18% by five species in Kano River. The distinct differences in fish abundance and richness between the two sections is ascribed to increased ecosytem productivity often associated with the inundated portion of dammed river. It is recommended that more water flow across the dams should be encouraged as a mitigating measure to boost basin sizes of the rivers at the below-dam sections. [ABSTRACT FROM AUTHOR]