The fate of iron (Fe) may affect that of phosphorus (P) and arsenic (As) in natural waters. This study addresses the removal of Fe, P, and As from streams in lowland catchments fed by reduced, Fe-rich groundwater (average: 20 mg Fe L(-1)). The concentrations of dissolved Fe (<0.45 μm) in streams gradually decrease with increasing hydraulic residence time (travel time) of the water in the catchment. The removal of Fe from streamwater is governed by chemical reactions and hydrological processes: the oxidation of ferrous iron (Fe(II)) and the subsequent formation of particulate Fe oxyhydroxides proceeds as the water flows through the catchment into increasingly larger streams. The Fe removal exhibits first-order kinetics with a mean half-life of 12 h, a value in line with predictions by a kinetic model for Fe(II) oxidation. The Fe concentrations in streams vary seasonally: they are higher in winter than in summer, due to shorter hydraulic residence time and lower temperature in winter. The removal of P and As is much faster than that of Fe. The average concentrations of P and As in streams (42 μg P L(-1) and 1.4 μg As L(-1)) are 1 order of magnitude below those in groundwater (393 μg P L(-1) and 17 μg As L(-1)). This removal is attributed to fast sequestration by oxidizing Fe when the water enters oxic environments, possibly by adsorption on Fe oxyhydroxides or by formation of ferric phosphates. The average P and As concentrations in groundwater largely exceed local environmental limits for freshwater (140 μg P L(-1) and 3 μg As L(-1)), but in streams, they are below these limits. Naturally occurring Fe in groundwater may alleviate the environmental risk associated with P and As in the receiving streams.

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Vegetation growth drastically influences water levels and flow patterns in lowland rivers. As soon as plants start to grow in spring, the flow resistance increases to reach a maximum in early summer.The universities of Ghent and Antwerp are conducting scientific research in the river Aa in Belgium in order to better understand the relation between aquatic weed growth and the hydraulic capacity of the river and to come to a more accurate determination of the different influencing parameters.Velocity measurements are performed in multiple cross-sections on a regular basis and studied. Discharge calculations are based on velocity measurements, which deliver important information about structural characteristics of the river, presence of vegetation, etc. Removing the vegetation allows for determining species and vegetation density as well as for studying the influence of plants on flow distribution and patterns. It is shown that the resistance coefficient, influenced by the amount of biomass, is an important parameter for the hydraulic characteristics of the river. The presence of macrophytes and their blockage characteristics are studied and a relation between discharge, biomass and resistance coefficient is set up.This leads to more reliable results in the application of hydraulic models for river management and river restoration purposes.