Transport and export of organic matter from the Colorado River, Grand Canyon during experimental high flow

Particulate organic matter transport in streams occurs predominately during floods. Although the same is likely true for big rivers, we lack detailed organic matter budget information for them. We measured longitudinal transport of particulate organic matter at 66 sites along the 360-km Grand Canyon reach at the peak of a 1165 m³/s experimental high flow. We additionally measured export through time at the end of the reach. Because of the bounded nature of this dammed river we can attribute all export to channel-derived material as opposed to tributaries or upstream segments. Particles <250 µm dominated transport; concentrations were about 10g ash-free dry mass (AFDM)/m³ until below the Little Colorado River where concentrations increased to 50 g AFDM/m³ showing much larger in-channel storage below this tributary. Concentrations did not decline downstream which agrees with theory that transport distance exceeded reach length of the entire canyon. Total export during the flood was 30,000 Mg AFDM, which corresponded to 950 g AFDM/m² scaled to the river channel. This large amount of exported organic matter exceeds our standing stock estimates of surficial benthic organic matter suggesting large buried reservoirs of organic matter were mobilized from sand bars during the flood.