Computation experiments found the effects of small-scale heterogeneities do influence estimates of aquifer--stream exchanges. The effects are nonlinear in the sense that simulated discharges from aquifers to streams are systematically lower than discharges produced by a base model parameterized with coarse effective K. Although aquifer heterogeneities are uncorrelated at local scales, they appear to induce coherent slow-paths in groundwater fluxes that in turn reduce aquifer-stream exchanges.