INVITED FACULTY TALK - Use Of Shields Stress Regime To Reconstruct And Forecast River Metabolism
Cronin, Greg 1 ; McCutchan, James H. 2 ; Lewis, William M. 3 ; Saunders, James F. 4 ; Pitlick, John 5
3 Presenting Author
1 University of Colorado at Denver
2 Center for Limnology, CIRES, University of Colorado at Boulder
3 Department of Ecology and Evolutionary Biology, Center for Limnology, CIRES, University of Colorado at Boulder
4 Center for Limnology, CIRES, University of Colorado at Boulder
5 Department of Geography, CIRES, University of Colorado at Boulder
Maps of Shields stress, which is proportional to the ratio of shear stress to substrate grain size and density, were made for a 1-km reach of the South Platte River below Denver (sand/gravel substrate) at specific stream flows. Field observations showed the entire bed moved above a threshold discharge capable of inducing critical Shields stress over most of the bed. Flows above the threshold discharge had a weak effect on respiration rate but strongly suppressed photosynthesis, thus lowering the P/R ratio. Rebound of photosynthesis rate, which occurred over 2-3 weeks, restored the P/R ratio to its earlier level. Given the relationship between threshold velocity and photosynthesis, it was possible to reconstruct daily photosynthesis and P/R ratio over the past 70 years. Reconstruction showed that water management has decreased the amount of time between excursions above the threshold velocity, thus suppressing degree of recovery between surges in flow; annual mean photosynthesis and P/R ratio are much lower now than prior to 1965. A similar procedure would allow prediction of the effects of various possible water management schemes on stream metabolism.