Stream restoration constrains fine particle retention within the hyporheic zone of urban streams

Stream restoration goals include reducing erosion and increasing hyporheic exchange to promote biogeochemical processing and improve water quality. Suspended fine particles (<100 m) exchange with transient storage areas near and within streambeds and banks. Fine particle deposition and retention directly impact carbon and nutrient cycling by supporting benthic and hyporheic metabolism, but accumulation of fine-particle deposits can impair metabolism by burying benthic biofilms and reducing streambed permeability. Little is known on how fine particle dynamics respond to stream restoration. We analyzed the transport and retention of water and fine particles at both the reach- and local-scale in a restored urban stream, 9 years post restoration. We injected a conservative solute and fine particle tracers under summer baseflow conditions and monitored the distribution of tracers between surface water, porewaters, and transient storage areas including biofilms, hyporheic zones, and slowly moving surface waters. Comparison of the results to a nearby unrestored stream demonstrate that the restored reach had an order of magnitude greater exchange of fine particles with transient storage zones, but 50% smaller rate of net particle immobilization. At the local-scale, particle immobilization in the hyporheic zone of both streams was similar, suggesting that restoration increased fine particle exchange with storage areas, but did not increase long-term particle retention. These results suggest that restoration structures successfully induced hyporheic exchange and maintained open pore spaces within the hyporheic sediments. However, the limited retention of particulate organic matter may reduce hyporheic biogeochemical processing by not providing sufficient organic carbon for heterotrophic microbial metabolism.