Seepage Erosion in the Luquillo Mountains, Puerto Rico, Relict Landscapes

Seminal research into geomorphic shaping of landscapes by emerging groundwater suggest that the relationships governing channel form and incision rate are distinct from other fluvial systems (Howard, 1995; Dunne, 1980, 1990). While some recent work has connected these models to natural settings (Abrams et al., 2009; Petroff et al., 2011), other studies have shown that seepage erosion may be erroneously invoked in landscapes shaped by flooding and overland flow (Lamb et al., 2006, 2007). We investigate the impact of seepage erosion on the geomorphology of a tropical mountain watershed, the Rio Blanco in the Luquillo Mountains of northern Puerto Rico. We focus on a population of amphitheater‐shaped channels that incise into deeply weathered saprolite profiles in the upland reaches of the watershed, where the river is disconnected from regional base‐level lowering (Brocard et al., 2015, 2016). We measured high rates of baseflow from springs feeding these catchments, despite their small drainage areas. We constructed an empirical model relating baseflow discharge to catchment area within the watershed. We quantified long‐term (103‐104 yrs) catchment erosion from 10Be in sediment and measured the short‐term volumetric flux of bedload transport at baseflow. Rates of transport and erosion in this group of channels scale linearly with the feeding drainage area and baseflow discharge. This finding supports a stream incision model proposed for groundwater‐driven channels (Howard, 1995). We propose that deepening saprolite and coincident entrenchment of the subsurface water‐routing system initiated seepage erosion and are actively expanding the headwaters in the Rio Blanco. Plain Language Summary The relationship between landforms and the processes that shape them on Earth can be used to identify the processes at work in places that can only be studied remotely, like the deep ocean or the surface of other planets. In the early 1980s, researchers recognized that river channels have different shapes depending on whether they are carved by water moving over the land surface or flowing underground. Early studies of groundwater erosion in sandbox models found that there is a unique relationship between erosion and the rate of groundwater flow into a channel that does not exist under other circumstances. However, this relationship has yet to be verified in natural landscapes. In this study, we measured the shape‐properties, groundwater flow, and erosion rates over short (h