Hydromorphological control of phosphorus in a large free‐flowing gravel bed river: the Garonne River (France)

Abstract

AbstractThe objective of this paper is to relate phosphorus (P) transport dynamics and hydromorphological characteristics of a large human‐influenced river, the River Garonne within a sector receiving the waste water of a sewage treatment plant for a population of 600 000. Two studies were conducted in 1997 and 1999 during two different hydrological conditions at low‐flow periods. The 1997 study was carried out on an 18‐km stretch with discharges varying between 33 and 53 m3/s and with very small fluctuations. The 1999 study concerned a longer stretch of 47 km, divided into four smaller reaches, and with discharges fluctuating rapidly from 40 to 108 m3/s.Downstream of the sewage treatment plant, total phosphorus (TP) concentrations ranged from 0.19 to 0.27 mg/L and were mainly in the dissolved form: between 60 and 78% of TP was dissolved reactive phosphorus (DRP). P concentrations were significantly lower upstream of the sewage treatment plant.By a mass‐balance approach, we estimated that the sewage treatment plant represents more than half the input (between 59 and 67%) of the studied sector. TP dynamic is linked to suspended solids for discharges above 60 m3/s.During established low‐flow period in the 1997 study (<60 m3/s), 22 and 27% of TP and DRP were retained by the river bed.In the 1999 study, under different low‐water period hydraulic conditions, we calculate that particulate P retention occurred in two reaches among the four under study and only for discharges below 60 m3/s.We show that for established discharges below 60 m3/s, there is an active uptake of transported P by functional compartments (i.e. the hyporheic zone and the periphyton). During the low‐water period with relatively high hydraulic fluctuations, and for discharges >60 m3/s, P retention is controlled as expected by suspended matter dynamics.We conclude that management of the hydrological regime can influence P retention during sensitive low‐water periods. Copyright © 2001 John Wiley & Sons, Ltd.