Constant rates of hyporheic exchange (“steady-state conditions”) are a common assumption when considering hydrologic residence time within the hyporheic zone. We have developed a simple extension of Danckwerts’ (1953) classic steady-state porous medium flow equations to describe the dynamic residence time distribution of transient (non-steady-state) flow in a hyporheic zone. As a simple preliminary test of the extended equations, we used salt tracer release experiments to measure hyporheic residence time distributions in an annular flume under steady-state conditions for three different surface water flow velocities. We then measured the flume’s dynamic hyporheic residence time distribution by varying surface water flow velocity over the course of individual salt tracer release experiments. Our results show that, as expected, residence time distributions shifted toward shorter residence times (and thus, hyporheic exchange rate increased) as surface water velocity increased. Using the steady state results, we estimated hyporheic exchange rates for each phase of the transient experiments and are now using the variation in hydrologic exchange rates to yield predictions of dynamic residence time distributions from our transient version of Danckwerts’ equations. Thus, these data provide a preliminary test for our simple, scalable mathematical description of dynamic residence time distributions under transient flow conditions.