Total body length and head capsule width (mm) were used as biometric characteristics in discriminating between size-frequency estimates of three Anacroneuria morphospecies, from 28 sampling sites along a tropical river subject to bimodal variations in water level in Central Colombia. A total of 344 specimens grouped into four size classes were collected. The highest abundance of Anacroneuria larvae was observed during the dry season and there was a higher abundance of small-sized individuals (body length <12mm). overall, the highest abundance of stonefly larvae was recorded between 1000 to 1600m.a.s.l. Conductivity and pH showed a significant correlation with the total abundance of preimaginal stoneflies. Plecoptera size distribution patterns could be useful in assessing which stonefly size-spectra are susceptible to disturbance in tropical rivers and in evaluating their population dynamics.