The hydrological research of the Loosdrecht Lakes and their surroundings over the period 1982-1990 was based on the integrated systems approach to surface- and ground water, with respect to their quantity and quality. The description of the water systems includes an evaluation of the structure and interactions of water systems in and around the lakes since the beginning of the Holocene until recent times, focussing progressively from the regional to the local systems and from the lake complex to the individual lakes. Quantitative research concerned: monthly and annual water balances for the complex of lakes and for individual lakes; numerical two- and semi-three-dimensional groundwater flow models; field experiments on turbulence and diffusion; and experimental laboratory studies on wind-driven currents. Qualitative research concerned: behaviour of environmental isotopes in groundwater; heavy metals in lake bottom sediments; phosphorus balances of the complex of lakes and of individual lakes for the existing and the simulated conditions; and remote sensing studies for the water quality pattern recognition. The interaction between the various water systems in the area has been influenced strongly in historic and recent times by human interference on both the local and regional scales of the water resources, e.g. through the reclamation of lakes, the extraction of water for public water supply, the regional transfer of water, the agricultural and urban activities etc. Two major remedial measures have been taken thus far: construction of sewer systems, and the supply with dephosphorized water from the Amsterdam-Rhine canal. These measures resulted in a decrease of the external P-load but did not result in a significant decrease in eutrophication as yet. Scenario's to improve this situation either pertain to tackling the fundamental causes of the eutrophication or to redressing their effects. The first category includes: drastic reductions in groundwater abstraction for public supply in the Gooi catchment area of the lakes and (partial?) flooding of adjacent deep polder areas to reduce the leakage losses from the lakes (both measures aim at restoration of the water balance). The second category involves less drastic water management alternatives for lake restoration: dredging P-enriched bottom sediments; reinforced flushing during summer with dephosphorized Amsterdam-Rhine Canal water; interception and rerouting of polluted inflows from the watershed; or additional P-removal from yet untreated inflows.
