The relative contributions of spatial and temporal dimensions of scale and their governance of ecosystem variation is an area of focus in ecology. From conservation, management, and monitoring perspectives, understanding assemblage fluctuations across temporal scales is of great interest. For example, most stream fish monitoring programs rely on data from single snapshots (e.g., one year) taken during a single time of year (e.g., summer low flows). Whether or not these snapshots are reflective of broader temporal variation is rarely tested. We surveyed stream fish assemblages at three longitudinally distributed locations in the San Saba River and the Llano River multiple times at two temporal scales. Surveys included annual samples of each site during the summers of 2021, 2023, and 2024 (i.e., annual scale), and season samples of each site during March, May, June, August, and November of 2024 (i.e., seasonal scale). We used standardized seining protocols during each survey, and we identified and counted all fishes collected. We used permuted multivariate analysis of similarity and fourth root-transformed abundance data to test for temporal differences in assemblage structure at the annual and seasonal scales. Results revealed no significant difference in assemblage structure among years or seasons for the six sites in the analysis. These findings suggest that inference gained from a single survey conducted during summer is representative of broader seasonal and annual time scales. Our results support the loose equilibrium theory that posits fish assemblages remain within some bounds around a central tendency with temporal fluxes in species abundances. However, environmental disturbances and anthropogenic alterations can push assemblages outside of their loose equilibrium state and into a state of directional change. Consequently, we suggest long-term monitoring programs be established in regions where future change is expected – and our results provide guidance for designing such programs.