Episodic wood loading in a mountainous neotropical watershed

a r t i c l e i n f o The Upper Rio Chagres drains 414 km 2 of steep, mountainous terrain in central Panama. A tropical air mass thunderstorm on 10 July 2007 produced a flood across the basin that peaked at 720 m 3 s − 1 at a headwaters gage draining 17.5 km 2 and 1710 m 3 s − 1 at a downstream gage draining 414 km 2. The storm also triggered numerous landslides in the upper basin, which facilitated the formation of large logjams along portions of the channel where transport capacity of wood was reduced by a change in channel geometry such as a bend or channel expansion. During field work in February 2008, we characterized three jams with surface areas of 400–2450 m 2 ; two of these jams resulted in storage of substantial (1100–8200 m 3) sediment wedges upstream. We returned to these sites in March 2009 to document changes in the logjams and sediment storage. Drawing on observations made in the basin since 2002, and site visits during 2008 and 2009, we suggest that jams such as these last two years or less. We propose that wood dynamics in the Upper Chagres alternate between brief periods of moderate wood load in the form of large logjams and much longer periods of essentially no wood load, a situation that contrasts with the more consistent wood loads in catchments of similar size in temperate environments and with limited studies of more consistent wood load in tropical catchments with no landslides. Relatively small, mountainous rivers exert a strong influence on local-, regional-, and global-scale hydrology, geomorphology, and aquatic ecology (Milliman and Syvitski,1992; Freeman et al., 2007), yet less is known about the basic physical and ecological processes and forms of these river systems than about larger, lowland river systems (Wohl, 2000). Knowledge of tropical mountain rivers is particularly limited by the relative dearth of field studies conducted in these environments thus far. The absence of field data makes it difficult to compare such basic attributes as rates and magnitude of change and temporal and spatial variations in process and form between tropical catchments and mountain rivers in other regions. Research on the geomorphic and ecological influences of wood in mountain rivers, for example, has grown exponentially during the past decade to encompass case studies from a wide variety of geographic settings, yet almost …