Net-spinning caddisfly (Hydropsychidae) larvae, at sufficient densities, have been shown to slow vertical water flux through streambeds. Therefore, net-spinning caddisflies may affect rates of water exchange between stream channels and hyporheic zones. We conducted a 19-day laboratory incubation experiment to assess whether hydropsychids and their net structures reduced hydrologic exchange, and how reduced exchange might vary among density treatments and over time. We constructed fifteen 56cm-diameter annular flume mesocosms containing 15cm of water atop 15cm of gravel and stocked each with one of five caddisfly densities. We conducted eight salt tracer releases in each mesocosm over the course of the incubation, and fit a first-order exponential decay curve to observed surface water salt concentrations. Likely due to algal growth in the mesocosm interstitia, the first order rate constant declined by ~25% over the course of the experiment, indicating reduced hyporheic exchange across all mesocosms. Notably, there was a greater reduction in hyporheic exchange at higher caddisfly densities. For instance, first order rate constants in mesocosms with the highest densities (~1250 caddisflies per square meter) declined by an additional ~6.5% relative to control (0 caddisflies) mesocosms. We conclude that ecosystem engineering activities of net-spinning caddisflies have the potential to influence hyporheic hydrology, with implications for stream biogeochemistry and ecosystem processes.