Rio Chagres Fieldwork

The Upper Rio Chagres watershed is truly unique and worthy of detailed study, interestingly having the distinction of technically draining into two separate oceans. Many of the difficulties encountered by the French in their failed attempt to build a sea level canal were the direct result of the unpredictable and extreme flooding of this river. Nearly one half of all water necessary for canal operations is derived from the Chagres basin, while occupying only one third of the total canal watershed area (Fig 1). In addition to the operational aspects of primary concern to ACP, a contingent of scientists and and engineers from many disciplines and countries have recently begun to study the Upper Chagres in considerable detail. Since that time an international symposium and an associated book have resulted.

Fig 1 Map of the Panama Canal watershed (shown by the yellow line) with the location of selected areas notated. Shaded areas within and near the watershed represent protected areas, reproduced and then modified from ACP (www.pancanal.com).

Excellent stream gage records exist just upstream of Madden dam thanks to the presence of the Canal. This gaging station defines the Upper Rio Chagres watershed, herein referred to as the URC. Several unique features are immediately revealed without having collected any specialized data. A typical year of runoff exhibits a pronounced dry season from late December until late April with little more than constant baseflow, as seen in Fig 2. Almost immediately after the wet season commences the hydrograph transitions to a state of flashy runoff. Digging a little further shows that the ratio of rainfall to runoff is typically highest at the beginning of the wet season. This is counter intuitive since one might expect the large cumulative volume of rainfall later in the wet season would act to saturate the soil column enhancing higher runoff efficiencies. Further October and November are the months that on average have the highest rainfall totals. Another interesting feature of the hydrograph that we immediately noticed were step wise increases in the base flow levels during the wet season. Are there some storage compartments within the watershed that upon filling provide an additional transport of water? What role do seasonal transitions play in runoff generation in the tropics? These are just two questions that must be answered to further our understanding of the important hydrologic processes in this tropical catchment.

Fig 2 Typical year of runoff in the Rio Chagres where the seasonality of runoff is clearly exhibited.

One area of particular interest to us at UConn has been runoff production mechanisms within the Chagres watershed. One method of complementing our studies from Gamboa has been to install stream gages at additional catchments within the Rio Chagres watershed as shown in Fig 3. Also shown are the drainage areas that each gage defines, an important consideration has been in scaling the drainage areas. The Rio Chagres and Piedras gages are operational stream gages owned by ACP. Due to the protected status of the watershed it is only accessible via helicopter or extensive hiking.

Fig 3 Location of stream gages within the Rio Chagres watershed and the catchment size that they represent. The Rio Chagres and Piedras gages are operated by ACP, the remaining gages are maintained by UConn. The dots represent the helicopter flight path from our August, 2005 trip.

Rio Chagres Book (We have two chapters):