The Hydrological Regimes of the Napo River Floodplains, Andean Amazon

Abstract:

Floodplains along the Napo River, a main Andean tributary of the Amazon, have variable and unpredictable flood regimes compared to those of the Central Amazon. The river is fringed by diverse fluvial and lacustrine ecosystems that provide important ecosystem services to the local inhabitants in one of the most biologically diverse regions on Earth. Little was known about the distribution of floodplains and their hydrological relationships with the river, yet such knowledge was needed as the river corridor faces increasing development pressure. From 2007 to 2013 we studied the variability in the main components of the flood regime across more than 100 sites using data loggers that continually recorded water levels, as well as rising stage samplers that collected flood water for major solute analysis to assess sources of water in the floodplains. We found a continuum of flood regimes ranging from deep and long-lasting flooding to shallow and short flooding with widely varying degrees of riverine influence. Also we found significant differences in depth of inundation among the three main reaches of the lowland Napo, with longer and deeper inundation in the uppermost reach. Seventy five percent of the floodplain study sites were directly influenced by Andean river waters, and approximately one fifth were only controlled by the river. Approximately 25% of the floodplain sites had flooding that was directly controlled only by local runoff or rainfall, although some were indirectly influenced by backwater effects caused by the rising river levels. In addition floodplains proximal (~1 km) to the river were significantly different than distal floodplains in terms of the depth and duration of inundation. These diverse ecohydrological patterns seem to be mediated by different vegetation types and non-linear connectivity levels among them and waterbodies. In contrast to the Central Amazon, the variable flood regimes of the Napo River floodplains preclude life history adaptations of organisms to long and predictable flooding and may favor species that can tolerate flashier inundation that act more as stresses than subsidies for much of the biota. This study provides fundamental information on the ecohydrology of floodplain ecosystems of the Western Amazon that is vital to protect and manage these ecosystems.