Reservoirs in agricultural catchments retain large proportions of inflowing phosphorus (P). However, the effects of reservoirs on the P cycle and related biogeochemical processes remain unclear. Therefore, this study investigated the degree to which a typical river-transition-reservoir in Southwest China retains both inflowing particulate phosphorus (PP) and dissolved total phosphorus (DTP) and various forms of P in sediments over different water seasons [normal-water season (NWS), low-water season (LWS), and high-water season (HWS)]. The proportions of inflowing PP and DTP retained were 37% and 27%, respectively. This result could be attributed to the absorption of DTP by the large load of intercepted sediment in the dam and the interception of PP itself. The rank of water seasons in terms of the proportion and load of inflowing TP retained was LWS (79%, 336 t P yr(-1))> NWS (21%, 43 t P yr(-1)) > HWS (4%, 27 t P yr(-1)), which might be due to the high P concentration 0.78 mg L-1 and long hydraulic retention time (HRT) 780 d during the LWS. In the long-term, there was a high rate of retention of bioavailable phosphorus (BAP) in sediments (63%). This result could be attributed to the combined effect of fine sediment particles and organic matter (OM). In addition, HRT (R-2= 0.89, p < 0.05) affected the retention of P more significantly than P concentration (R-2= 0.56, p < 0.05). Dam interception during the LWS resulted in high BAP contents (280 mg kg(-1)) in sediments, high P concentrations (0.78mg L-1), and weak hydrodynamics (HRT: 780 d) in overlying water. Therefore, further regulatory measures are urgently demanded during the LWS to prevent reservoir algal blooms. (C) 2021 Elsevier B.V. All rights reserved.