Peer-reviewed paper published on Marañón hydrology
Marañón Project hydrologist Alice Hill, together with co-authors at US Geological Survey and University of Colorado, published a paper stemming from data collected on the 2015 expedition. The paper, titled "Clarifying regional hydrologic controls of the Marañón River, Peru through rapid assessment to inform system-wide basin planning approaches" aims to identify what controls dry season baseflow on the Marañón. The goal of the paper is to clarify what controls the hydrology across the basin so that future flow changes can be anticipated in light of sustainable resource development of the river corridor. You can find the whole open-access paper here, or keep reading for the key findings.
Alice finds that huge alpine wetland systems (locally called "punas") are critical storage reservoirs of wet season rain. Alice estimates that the vast majority of the flow at Balsas - about 200 m3/sec (a LOT of water) -- is wet season rain that saturate the wetlands and gradually seep out during the dry summer. Accumulated over the dry season, this means that the punas store about the same volume of water as would be needed to supply the 10 million person population of Lima for 2-3 years-- this is a seriously big reservoir!
This brings to bear questions about the long term sustainability of the puna ecosystems. There are land use change considerations (punas being transformed into agricultural areas) that warrant attention. Also, other studies in the region show a connection between the dwindling glacier melt inputs to the punas and decreasing puna size and extent. So, while the snow and ice melt don't appear to directly play a role in the Marañón's flow patterns, it does look like they have an indirect effect insofar as they could affect the size of the puna reservoirs that are relied on for summer flow.
Groundwater and punas are the story in the upper corridor where most of the dams are proposed. The hydrology switches to a rain dominated river once we get into the jungle pongos. Isotope data indicates most of the water coming in from the lower elevation big tributaries marches its way up from the Amazon forest, hits the pongos topography and dumps out much moisture that feeds the mainstem Marañón.