The 21st Century has experienced many natural disasters such as floods, tsunamis, landslides as a result of the looming effect of climate change. This calls for innovative ways through which we can model and manage natural systems e.g. rivers, streams, lakes. The advent of GIS and Earth Observation (EO) in recent times provide us with endless possibilities for modelling, forecasting and predicting phenomena of water bodies in time and space with reasonable accuracy. ILWIS Open is free and open source GIS/EO software that presents us with some of these possibilities but however lacks water quality analysis of rivers and streams. This work, in contribution towards a solution, developed a plug-in ILWIS Open to handle water quality modelling in rivers and streams. The plug-in is based on the USGS Transient Storage Solute Transport (OTIS) and the USGS Full Equations (FEQ) models. The plug-in can stimulate hydraulic parameters that influence transient storage under steady and unsteady conditions. Professional and scientific interest for using this plug-in are already well noted with the Regge and Dinkel water board (http: //www.wrd.nl) and 52North (http: //52north.org/).