Irrigation canals transport irrigation return flow, including water, sediment, and nutrients, back to surface water bodies. Phosphorus transformations during canal transport can affect both P bioavailability and the best management practices selected to minimize P inputs to waters of the US. The objective of this study was to determine P retention in an irrigation canal. Dissolved reactive P (DRP) concentrations in canal waters were increased from 0.08 mg L^-1 to 0.25 mg L^-1 by constantly injecting a P solution for 2.5 hrs. Br was used as a conservative tracer to determine dilution effects. Water was sampled at 10 min intervals 90, 180, 360, 760, and 1540 m downstream from the injection site. When at steady state, P concentration only decreased by 5% over 1540 m, equating to a P uptake length of 16.1 km, which is 2 to 3 orders of magnitude greater than P uptake lengths typical of natural streams. The linear P uptake rate was 1.1 μg P m^-1 s^-1. Longer P uptake length and lower uptake rate compared to natural streams may be a result of lower algal growth and higher water velocities.