Why Canada's Rupert Bay looks as though rivers are pouring tea

The brown swirls entering Canada's Hannah and Rupert bays can look like an enormous cup of tea viewed from space. NASA Earth Observatory's October 6 account explains that the comparison has a chemical basis. Rivers draining forests and wetlands carry dissolved organic substances that colour the water before it reaches the coast.

Leaves, bark, roots and soils release material as they decay. Some of those compounds absorb blue and ultraviolet light, giving the water a brown appearance. This coloured dissolved organic matter includes substances related to tannins and lignins. The colour does not, by itself, prove that an industrial spill has occurred.

Landsat 9 recorded the scene on September 27, showing river water spreading across shallow mudflats and mixing with James Bay's darker blue waters. A second source of brown is suspended sediment. Tides and river currents stir fine particles from the bottom, creating paler plumes that mingle with the more deeply stained water.

The coast is also changing on a much slower timescale. Land continues to rise through postglacial rebound after the removal of ancient ice-sheet weight, while rivers deposit sediment. Together, the broad shallow coastline and continuing deposition help maintain the mudflats seen in the satellite image. Colour is therefore only one part of a larger landscape process.

On land, branching green patterns reflect differences in vegetation and drainage. More wooded stream corridors appear darker, while poorly drained peatlands have less extensive tree cover. The image connects the chemistry of the rivers with the forests, soils and wetlands through which their water has travelled.

NASA's account also discusses the region's long Cree history and the importance of beavers in local culture and trade. The useful scientific lesson is that a striking colour needs an explanation grounded in context. Several natural processes can share the same photograph, and distinguishing dissolved material from floating sediment helps readers understand what the satellite actually sees.