Algal Blooms Algal blooms have occurred in both rivers for some years. However, since 2016 these blooms have become much more severe and prolonged, turning the water anything from a light green to dark brown in colour. The problem is caused by a combination of sunlight, low flows and elevated levels of phosphate in the water. Read more on the causes and problem with algal blooms along with what the Foundation is doing to tackle this issue.