Within the Poodří nature reserve and the Odra River old parallel channel, Slaňáky, various forms of phosphorus were investigated due to increased high concentrations of total phosphorus (Ptot) in the surface water. Surface water samples were subjected to chemical analyses using absorption spectrophotometry (Ptot and P-PO₄^3- ) and accompanying parameters (t, O₂, pH, Cond, TSS ) were measured. Next, grain size analysis of the sediment and sediment analyses using X-ray fluorescence were carried out (P, Fe, Al, Ca, Si, K, Mn, S, etc.). The seasonality of Ptot concentrations in the water was not confirmed, the reservoir behaved as a pool. The concentrations of the dissolved inorganic form P- PO₄^3- were low when compared to Ptot, thus suspended phosphorus prevailed. This may be related to the low depth of the given water body, where stratification does not occur in contrast to deep water bodies. However, sediment resuspension is frequent, and phosphorus largely accumulates. The concentrations of Ptot in the sediment ranged around 4900 mg/kg, where the inorganic form prevailed (87.24 – 93.73 %) over the organic (6.27 – 12.76 %). At the same time, the sediment samples were abundant for sand fractions (2 – 0.03 mm), 47.86 – 60.89 %, and finest silt and clay (< 0.063 mm), 33.07 – 44.60 %.