Zero altitude determination for hydrometric stations in the Brazilian Amazon. Application to the water surfaces of Rios Negro, Solimões and Amazon
The altimetric relation of a hydrometric station to the General Levelling mark is usually performed in order to know, when there are several stations, the slope of the water line. Until the end of the 2000s, Amazonian hydrometric stations were not related because the lack of roads was a major obstacle to classical direct levelling. However, the end of this decade has seen the expansion of satellite altimetry and more particularly the development of the method using GPS (Global Positioning System) associated with the use of the geoid model EGM08. This new technique is particularly valuable for level gauging stations in the Amazon Basin. Here we have treated hydrometric stations of the Amazon from its mouth to the Brazil/Peru border and those of the rivers Negro and Solimões in their Brazilian part. Average slopes of these reaches decrease normally for the Rios Solimões and Amazon, whereas for the Moura/Manaus reach on the Rio Negro, the abnormally low slope (6 mm•km‐1) may cause inverted flow. The lower Amazon is connected to the altitude given by the trace of orbit 202 of the Topex-Poseidon and Jason-1 satellites, but unfortunately we only have one year of gauging readings for this reach. Comparisons of the water slopes with other rivers (Volga, Danube, Nile, Mississippi and Rhine) indicate that the slope of the Amazon is, with an average of 21 mm•km‐1 from Tabatinga (Brazil/Peru border) to the Atlantic Ocean (a distance of 3000 km), two times less than that of the Volga (which has the lowest water line slope among the five rivers studied). This, coupled with the fact that the bed of the Amazon is, for about 2000 km from its mouth, about forty meters below the level of the ocean, would lead us to believe that the river's flow is propelled by water arriving from upstream, from the Andes.