We measured the hyporheic residence time distribution in a 2nd-order mountain stream at the H. J. Andrews Experimental Forest, Oregon, and found it to be a power-law over at least 1.5 orders of magnitude in time (1.5 hr to 3.5 d). The residence time distribution has a very long tail which scales as t^−1.28, and is poorly characterized by an exponential model. Because of the small power-law exponent, efforts to characterize the mean hyporheic residence time (t_s) in this system result in estimates that are scale invariant, increasing with the characteristic advection time within the stream channel (t_ad). The distribution implies the hyporheic zone has a very large range of exchange timescales, with significant quantities of water and solutes stored over time-scales very much longer than t_ad. The hyporheic zone in such streams may contribute to short-time fractal scaling in time series of solute concentrations observed in small-watershed studies.