In 2024, Ireland's Environmental Protection Agency (the EPA), in collaboration with Compass Informatics, began creating a new national river network using highly accurate vector data provided by the national mapping agency Tailte Éireann. This will replace the current river network based on 50,000 scale data. The existing river network contains around 85,000 Km of water channels, whereas the new dataset is almost 125,000 Km in length. However, the new dataset contains gaps where water flow is not visible, such as through culverts under roads, and lacks essential attributes required for environmental monitoring such as flow direction and stream order. Currently, a project is underway to join gaps in this dataset, create flow lines through waterbodies such as lakes and transitional waters, add missing features, and connect the network through groundwater aquifiers. An on-line GIS editing portal was developed to support the project using open-source software, which will be the focus of the talk. A front-end web GIS was developed using OSGeo projects OpenLayers and GeoExt, and other open-source geospatial JavaScript libraries including cpsi-mapview. The backend uses MapServer and Python web services, building on numerous open-source geospatial libraries including NetworkX, Shapely, skgeom, and two libraries created by Compass Informatics: Wayfarer, a Python library for analysing geospatial networks, and Cascade, a Python library for applying stream orders to vector networks. Cascade will be released as an open-source library as part of this project. Upon completion, the new river network will enhance the EPA's modelling and assessment capabilities across water and environmental domains. This includes sediment and flow modelling, catchment assessments, water quality monitoring, and delineation of river waterbodies. The new network will benefit many other organisations for applications such as fisheries and flood management and become a key component of Ireland's national data infrastructure.