Environmental DNA (eDNA) analysis has become increasingly popular in freshwater ecological studies, but relating eDNA samples to source organism locations continues to pose a challenge in lotic systems due to downstream flow. We hypothesize that particle size distribution (PSD), the size range of eDNA fragments, may yield insight on this issue, since larger fragments are removed more quickly by deposition and degradation, while smaller fragments, produced by degradation of larger fragments, remain suspended in the water column for longer periods of time, and are therefore transported longer distances. However, previous studies have shown that PSD may vary by species, possibly due to behavioral factors such as proximity to benthic habitats or physiology. We compared PSDs in the water column between two species in the same genus: the benthic Pecos gambusia (Gambusia nobilis) and the pelagic Western mosquitofish (Gambusia affinis). We collected surface water samples at 7 sites along the Lower Pecos River, Texas, and segregated eDNA into small and large size classes via sequential filtration. Next, we applied species-specific qPCR assays to quantify eDNA of our focal species. We then compared the quantity of DNA on each filter size to determine the ratios of large to small DNA fragments in the water column for each species. Preliminary findings show that PSD differs between sites and between species. Further analysis of PSD will improve our understanding of downstream eDNA transport in lotic systems and inform future research related to predicting source organism location based on the PSD of eDNA samples.