Gravity-driven, dead-end ultrafiltration of surface water, performed at ultra-low pressures without backflushing, cross-flow or chemical cleaning, results in flux stabilization during extended periods of time. This principle can be used for direct ultrafiltration of surface water without an external energy supply and low maintenance for decentralized drinking-water treatment. Here we investigate the mechanisms of membrane fouling and flux stabilization during gravity-driven, ultra low-pressure UF (ULP-UF). The impact of natural organic matter on the structure and resistance of the fouling layer is studied for different types of water, namely river water, pre-treated river water, river water with addition of sodium azide, aerobic diluted wastewater, diluted wastewater with a low dissolved oxygen content and river water spiked with humic acids or kaolin. Our results show that the deposition of non-dissolved material, biologically induced structural changes in the fouling layer and development of irremovable fouling are three major processes determining the fouling and flux stabilization in ULP-UF. The biologically induced structural changes in the fouling layer lead to heterogeneous structures and channel networks there and cause a decrease of the specific resistance of this layer over time. Flux stabilization occurs when the decrease of the resistance due to structural changes in the fouling layer balances the increase of resistance due to deposition and irremovable fouling.