This thesis presents an experimental study, both in the field and laboratory to cast more light on the primary role of the river floodplains in releasing and/or removing total-P to/from the in-stream load, under high runoff and flood conditions, by investigating the soil total-P spatial and vertical deposition patterns and topsoil erodibility, along the three (3) main river sections (e.g., headwaters, transfer and deposition zones) of an agricultural watershed, such as the Turkey River (TR). In soils, phosphorus, P, primarily exists as sediment-bound and less often as dissolved. During wet hydrological years, soil erosion and surface runoff are the main P release and transport mechanisms, while during dry hydrological years, P leaches to the deeper soil levels and is transported to freshwaters through groundwater discharge. In between the upland areas and the river network, there is a buffer zone, known as floodplain that regulates the flux exchanges between these two watershed components. Floodplains play an essential role in the riverine system health by supporting important physical and biochemical processes and improving the water quality downstream. These characteristics have led to the conclusion that floodplains primarily act as sinks for P. However, floodplains are subject to erosion as well, where soil particles along with the attached P are removed from the topsoil or enter re-suspension, under high runoff and flood conditions. The study provides an insight into the soil total-P deposition patterns across the floodplains of five (5) identified field sites and couples them with topsoil erodibility to eventually address the research objectives, which can be summarized as follows: (i) investigation of the soil total-P spatial and vertical variability across the floodplains along the main river zones and development of relationships between P variability and soil physical properties (e.g., soil texture); (ii) identification and characterization of the soil total-P deposition patterns across the floodplains (e.g., short- vs. long-term P deposition areas); and (iii) comparisons of the soil total-P concentrations and critical shear stresses among the main river zones and determination of their primary function either as P sources or sinks, under high runoff and flood conditions. Following that line of thinking, this research results comprise of three (3) parts, each one addressing a specific objective. The first part of the results includes the soil texture and total-P concentration analyses of the extracted soil profiles to identify the soil total-P spatial and vertical deposition patterns across the floodplains, as well as, to investigate the total-P variability with respect to soil physical properties (e.g., soil texture). The second part of the results focuses on investigating the role of topography (e.g., flat vs. ridge vs. swale land surfaces) and flood characteristics (e.g., frequency, magnitude, duration) in soil total-P spatial and vertical deposition patterns across the river floodplains to understand the time-scale nature of the P storage. The last part of the results presents the experimentally determined topsoil critical shear stress values and erodibility rates to characterize the floodplains’ primary function, based on their location along the three (3) main river zones, either as sources or sinks for total-P, during high runoff and flood conditions. Overall, the results of this research show that (i) the total-P concentration in soils is tightly related to the fine particle content and monotonic linear relationships can be established between the two variables. In other words, the higher the fine particle content, the higher the total-P concentration in soils; (ii) a mixture of two normal distributions fit the log-transformed soil total-P concentration data of each field site considered in this study. The fitted distributions successfully capture the two peaks of the soil total-P concentration data correspond to the lower and upper floodplain terraces; (iii) the lower floodplain terraces (e.g., 2- and 5-year floodplains) are characterized by significantly lower soil fine particle percentage contributions and total-P concentrations compared to the upper floodplain terraces, at a 5% confidence level. These patterns can be attributed to the fact that the lower floodplain terraces are frequently flooded and/or under inundation compared to the upper floodplain terraces and thus part of the fine particles along with the attached P are regularly winnowed away. Therefore, the lower floodplain terraces can be considered as short-term P storage means, in between two consecutive major flood events, while the upper floodplain terraces act more as long-term P storage means; (iv) there is a longitudinal increase in the topsoil critical shear stress values, which follows the increase in the fine particle content reconfirming the principle that the more the fine particle content in soils, along with the existence of vegetation with dense, well-developed root systems, the more resistant to erosion are the soils. From a soil erodibility perspective, the floodplains along the headwaters zone can be considered as major fine sediment and total-P sources contributing to the in-stream loads, while the floodplains along the deposition zone primarily act as sinks for fine sediment and total-P. As far as the role of the floodplains along the transfer zone, they can be considered as sinks for fine sediment and total-P during low magnitude runoff and flood events (e.g., 2-; 5-; and 10-year return periods), while during higher magnitude events, they act as sources releasing fine sediment and total-P; and (v) topsoil samples characterized by dense, well-developed root systems fall approximately along a trend line that follows almost a parallel pattern with the trend line for the topsoil samples without dense and/or well-developed root systems. The existence of dense, well-developed vegetation root systems to topsoil consistently increases its critical shear stress threshold (e.g., > 1 Pa) and thus its ability to resist erosion.