Basement membranes are an ancient form of animal extracellular matrix. As important structural and functional components of tissues, basement membranes are subject to environmental damage and must be repaired while maintaining functions. Little is known about how basement membranes get repaired. This paucity stems from a lack of suitable in vivo models for analyzing repair. Here we show that Dextran Sodium Sulfate (DSS) directly damages the gut basement membrane when fed to adult Drosophila. DSS becomes incorporated into the basement membrane, promoting its expansion while decreasing its stiffness, which causes morphological changes to the underlying muscles. Remarkably, two days after withdrawal of DSS, the basement membrane is repaired by all measures. We used this new damage model to determine that repair requires collagen crosslinking and replacement of damaged components. Genetic and biochemical evidence indicate that crosslinking is required to stabilize the newly incorporated repaired collagen IV rather than to stabilize the damaged collagen IV. These results suggest that basement membranes are surprisingly dynamic.