The Río Chagres, Panama

The Republic of Panama occupies about 77,382 km² and, despite its relatively small size, displays a remarkable degree of physical and cultural diversity. Part of the country's physical and biological variety can be attributed to its absolute location within the tropics. Panama's relative location, however, is equally responsible for both the physical and cultural complexity of the country. As the land bridge between the Americas and the major link between the world's two largest oceans, Panama is the crossroads of the western hemisphere. The country's position relative to the continents and oceans constitutes its most important situational advantage. This geographic analysis focuses on the physical geography of Panama, with the goal of providing an overview and larger context for the other papers of this volume. It considers the implications of its relative location and summarizes the physical geography of Panama in general terms drawing from geomorphology, climatology, and biogeography. Although the climate is tropical, Panama experiences significant climate diversity over relatively short distances. Panama also has considerable relief within its comparatively small territorial extent. Elevation differences and associated temperature and precipitation patterns produce distinct vegetative regimes and contribute further to the country's biodiversity. The country's most celebrated resource, however, is its unique location at the intersection of the western hemisphere's continents and oceans.

The Panama Canal Watershed is a hydrologically complex, ecologically diverse managed natural-artificial managed water resource system composed of many sub-basins, rivers, and dammed lakes extending across 2,982 km² on both sides of the Panama Canal. The upper Río Chagres basin is the largest headwater unit in the watershed, occupying about one-third the total area but supplying almost half of the water needed for canal operation. Total runoff across the watershed is such that there is inadequate input of water during low-flow years to provide the all of 4.1×10⁵ m³ of water needed to operate the canal, generate electricity, and provide public drinking water. At present, forest preservation is arguably the most important water resources management issue for the Panama Canal Watershed because deforestation causes enhanced soil erosion and reservoir sedimentation and also strongly affects the timing of the runoff. Overall, prospects for the long-term sustainability of the water resources of the Panama Canal Watershed are good, given the large percentage of land within the watershed that is federally protected. However, the challenge for the immediate future is to develop and enforce a consistent set of informed and enforced land management to ensure that the water resources will continue to be available to operate, if necessary, and expand the Panama Canal for the benefit of Panama and the world.

This paper reviews the history of settlement in Panama and then summarizes the important findings of the Panama Canal Watershed Natural Resources Monitoring Project, a multidisciplinary study was conducted between 1996–1999.

The Panama that geologists see today is a young landscape that in form comprises a reclined S-shaped, generally E-W oriented isthmus produced through complex geotectonic processes that created and assembled a diverse suite of geological units since late Cretaceous/early Tertiary time. The geological development of Panama is a consequence of the relative motions of the North and South American continental plates and four oceanic plates over the past 150 million years, and is a part of the larger story of the tectonic development of the Caribbean basin and Central America. The igneous rocks that comprise much of present-day Panama formed during Tertiary time as an oceanic plateau associated with the Galapagos plume and from an oceanic volcanic island arc complex that presently extends from the southern margin of Nicaragua to the northwestern Colombia. About 10 million years ago, the Panama-Costa Rica arc began to collide with northwestern South America, cutting off the deep-water circulation between the Pacific and the Caribbean. Development of Central America was completed around 3 million years ago, creating the land bridge between North and South America and terminating the shallow marine circulation between the Pacific and Caribbean.

The geological basement of the upper Río Chagres basin (RCB) is primarily a mixture of Cretaceous to Upper Tertiary age volcanic and intrusive rocks. Exposed rocks consist of highly deformed mafic basalts, basaltic andesites, gabbros, diorites as well as chemically more evolved granodiorites, tonalities, and granites. Ultramafic rocks, that would provide evidence for an oceanic basement/lithospheric mantle basement to the RCB, are absent. Primary stratigraphic relations and contacts are generally obscured, either tectonically or by deep weathering. Most rocks, in particular the volcanics, volcaniclastic sediments, and granites are all strongly deformed and chemically altered. Mafic and granitic rocks have distinct weathering characteristics that influence stream channel morphologies throughout the RCB. The mafic complexes are most resistant to weathering and mechanical erosion, producing narrow river channels, rapids, and deeply cut gorges. Granitic lithologies are most easily weathered and generate straight and wide river courses. Massive altered basalts are intermediate in their style of weathering. Dike swarms crosscut all lithologies and strongly influence river channel form and orientation. The geochemical composition of the rocks suggest that the majority are derived from extensive volumes

To support a number of projects focused on diverse biological and physical science aspects of the upper Río Chagres basin, a detailed stream network was extracted from digital elevation data obtained by interferometric radar survey. The elevation data represented the bald earth surface plus a forest canopy of varying height. Therefore, different algorithms for stream network extraction were qualitatively evaluated in terms of their capability to extract accurate stream locations from this challenging type of elevation data. The programs based on a shortest path algorithm and an imposed gradients constraint provided stream locations that were closer to on-ground GPS measurements than the tools based on depressions filling and iterative linking. The influence of different spatial resolutions on network structure and orientation was also explored.

Understanding the relationship between rainfall, and stream flow in mountain terrain requires the quantifying of rates of water movement into and through regolith covered hillslopes. General theory holds that infiltration rates in humid tropical are higher than rainfall intensities so surface runoff is minimal. However, soil profile characteristics can vary significantly on a hilslope, with concomitant changes in soil hydrologic characteristics. The pattern of soils within two small first order drainages was evaluated within the upper Río Chagres basin. Two main influences on soil distribution were identified. Mass movements primarily translational sliding and treefall result in stripping of the upper soil horizons and exposure of weathered saprolite. Soils forming in the deposits are characterized by higher infiltration rates and a more uneven surface topography than the stable soils. A catenary relationship was also observed with stable, oxidizing soil profiles in upper slope positions and reduced (gleyed) soils at the outlet of the drainage basin.

Soil hydrologcal processes determine how precipitation is partitioned into infiltration, runoff, evapotranspiration, and ground water recharge in the upper Río Chagres basin. The focus of this study is to investigate the soil hydrological processes by which precipitation excess on first order drainage basins enters the streams feeding the upper Río Chagres and its major tributary rivers. Infiltration rates, water retention curves, and water repellency of surface soils have been measured. These measurements together with the soil morphological observations by Harrison et al. (2005, Chapter 7) and hydrological observations by Calvo et al. (2005, Chapter 9) and Niedzialek and Ogden (2005, Chapter 10) are used to formulate a comprehensive conceptual model of runoff production in the upper Río Chagres watershed.

Annual runoff hydrographs recorded by the Panama Canal Authority in the upper Río Chagres basin indicate several peculiar features. First, the annual hydrograph is strikingly seasonal, with very few signs of direct runoff and a continuous decay in base flow during the dry season. Secondly, there are signs of anomalously high runoff production efficiencies early in the wet season. Thirdly, the base flow from the catchment exhibits up to three different “quasi-stable” base flow discharges as the wet season progresses. This study examined runoff generation in the upper Río Chagres basin using the US Department of Agriculture, Natural Resources Conservation Service ‘Curve Number’ (CN) methodology. Specifically, variation curve of the CN was analyzed using rainfall and runoff observations from the basin. Results indicate significant influence of seasonality on the CN. Furthermore, there are significant inter-seasonal changes in the CN that invalidate the applicability of the CN approach in this tropical watershed.

Runoff production in tropical watersheds is governed by a wide variety of potential sources and there have been few rigorous studies to date. The 414 km² upper Río Chagres basin offers a unique opportunity to better understand the runoff production mechanisms in tropical watersheds through data analysis and modeling with rainfall and runoff data. Flow data and tipping bucket rain gage data are available at both the basin outlet (Chico gage) and for an 80.6 km² internal basin location (Piedras gage). Modeling is performed using the Sacramento Soil Moisture Accounting Model (SAC-SMA), calibrated using data from 2000 and verified using data from 2001. The flood event of 28–31 December 2000 was examined in detail. Data analysis and modeling reveal critical threshold storages in the catchment, and anomalously high runoff production at the start of the wet season. This conclusion is supported by field studies that reveal evidence of high storage capacity and dry season water repellency. Observation of discrete quasi-stable baseflows in the upper Río Chagres is not seen in the internal Rió Piedras drainage, which is shown to exhibit ephemeral behavior year-round. New data collection and monitoring is proposed for the upper Río Chagres catchment, including measurements of rainfall above canopy, cloud stripping, stemflow, throughfall, soil moisture, interflow, and overland runoff measurements.

The upper Río Chagres basin drains 414 km² of mountainous topography in central Panama. Hillslopes exceed 45° in over 90% of the basin and are highly dissected by a predominantly bedrock channel network. A variety of igneous bedrock lithologies are exposed discontinuously along the channels, which both follow and cut across geologic structures. Local changes in channel gradient and geometry occur where the channel crosses a more resistant bedrock unit, but the longitudinal profile as a whole is concave. Coarse sediment is introduced via tributary channels and landslides. The basin has high values of discharge per unit area. Channel geometry, bed grain-size, velocity, and discharge were surveyed for 40 short (50–200 m-long) channel reaches throughout the basin to examine how downstream hydraulic geometry relations and other channel characteristics (i) compare to those from mountain drainages in temperate regions, and (ii) reflect site-specific variables versus variables such as drainage area and discharge that change progressively downstream. Downstream hydraulic geometry exponents are 0.36 for hydraulic radius, 0.43 for channel top-width, and 0.24 for mean velocity. These values are similar to the average values for rivers worldwide, despite the inclusion of step-pool, pool-riffle, and bedrock gorge reaches in the data set. The presence of strong downstream hydraulic geometry trends indicates that hydraulic driving forces sufficiently exceed resisting forces to override specific lithologic influences on channel width and depth when averaged across sub-basin to basin scales. Poor correlations between grain size and drainage area, discharge, or reach gradient and between reach gradient and drainage area or discharge suggest that landslides and bedrock lithology locally control grain size and reach gradient.

The upper Río Chagres basin is a significant source of water for operation of the Panama Canal, producing a disproportionate share of the total runoff to the canal and also serving as the source of metropolitan drinking water for Panama City and Colon. To better understand the distribution of rainfall in the upper Río Chagres watershed, a study was performed using rainfall observations from both ground- and space-based platforms in a geospatial statistical framework. The ground-based data are from two rain gages with long-term records and five additional gages installed in 1998. Given the significant topographic relief, and the sparsely distributed network of rain gages, it is likely that the spatial variability of rainfall is not accurately measured by this rain gage network. This paper asks two questions: Does a significant relationship exist between precipitation and elevation? Can the fraction of total catchment's rainfall coverage for storm events be determined? Three methods are to answer these questions ussing TRMM (Tropical Rainfall Measurement Mission) data products. The TRMM satellite provides 4 km² resolution PR (Precipitation Radar) and much coarser TMI (TRMM Microwave Imager) observations multiple times per day. The instruments observe swath widths of 220 and 758 km respectively. Full basin coverage of a spatially distributed rain field observation is possible at PR resolution, allowing for determination of the areal extent of rain. Moreover, TRMM data can be used to develop a statistical model of the spatial variability of rainfall in an effort to make basin average estimates. Thirdly, available TRMM rainfall estimates are used directly, in validation of rain gage data.

Tree species composition at two sites in the upper Río Chagres basin of central Panama was evaluated using rapid inventory methods. At each site, two 40×40 m quadrats were demarcated, and each was thoroughly searched for tree and shrub species. The 40×40 m quadrats had a mean of 155 species each, and the four pooled had 285 species; 29 other species were noted along trails near the survey plots. These inventories were compared to 81 others within the Panama Canal Watershed, and forest composition and diversity was evaluated relative to mean dry season duration. The upper Río Chagres sites have high rainfall and are rich in tree species relative to most of the area; the only area with higher diversity is the Santa Rita Ridge, along the Caribbean coast, which is even wetter. Many tree species are restricted to these wet areas of central Panama, not occurring in drier areas of the Pacific slope or central Panama.

High spatial resolution airborne and satellite sensors have been used with varying degrees of success to measure deciduousness, canopy structure, and light interception, and to identify tree species in the Panama Canal Watershed. Results reported to date indicate that remotely sensed data have a high degree of accuracy in measuring deciduousness and leaf density in the upper canopy, but less accuracy than in temperate systems in measuring canopy light interception for semi-deciduous lowland tropical forests in the canal watershed. Of particular relevance to evergreen forests like the upper Río Chagres basin, this work examines whether high spectral resolution data, like that collected by the HYDICE system, can separate canopy species based on hyperspectral signatures in the 0.4 to 2.5-µm wavelength region. If a few well-selected spectral bands could accurately differentiate species, tropical canopies in remote and rugged terrain could be mapped using relatively simple, but optimized sensor systems.

In a recent study examining the degree to which tree species composition differs among rainforest sites (i.e., β-diversity), Condit et al. (2002) found that plots in the Panama Canal Watershed separated by 50 km were more highly differentiated than plots in western Amazonia separated by nearly 1400 km. The high β-diversity of trees in Panama was attributed to sharp environmental gradients between the Atlantic and Pacific coasts. However, the pattern may also result from Panama's history as a land bridge over which floras from Central America and South America mixed during the Great American Biotic Interchange (GABI) roughly 3 million years ago. Under this scenario, it would be expected that wetter Panamanian forests would contain more trees of South American origin, whereas drier Panamanian forests would have more trees of Mesoamerican origin due to the historical prevalence of dry habitats in Mesoamerica. This idea was tested by quantifying the geographic distributions of 714 tree species found in three sites in the Panama Canal Watershed, which represent a gradient in annual rainfall. Nearly identical proportions of geographic representation of trees among the three sites, with species distributions of ca. 15% Mesoamerican, 17% South American, 9% Panama endemic, and 59% widespread. These data do not support the biotic interchange hypothesis. However, this analysis found that 433 of the 714 tree species (61%) have a cross-Andean distribution, which suggests that these tree species may be sufficiently old to have participated in the GABI.

The avifauna of Panama may be better documented than that of any other country in Central America. Museum expeditions over the past 100 years have yielded a wealth of traditional specimens and an excellent understanding of bird distribution and occurrence across the country. Unfortunately, most of these expeditions were mounted in the first half of the century well before the importance of preserving tissue samples had been realised. Consequently, the world holdings of avian tissue from Panama are grossly inadequate. A compilation of the world holdings of avian tissue from Panama is presented which illustrates the need for continued general collecting throughout Panama. Recent contributions made during a recent field visit to the upper Río Chagres basin in 2002 are listed and guidance provided about where future collecting is needed.

This paper presents an approach for assessing the regional importance of landslides based on conventional daily-discharge and daily-sediment data from sub-basins within the Panama Canal Watershed. In many wet mountainous regions, sediment yields are controlled by both surficial erosion and deep, landslide erosion. Landslides require that rainfall (and by inference runoff) exceed a threshold. Runoff can be used to derive a parameter termed ‘landslide days’, with a suitable correction factor, to account for evapotranspiration, infiltration, and rainfall patchiness. The approach developed then uses runoff as the driver for a simple surficial-erosion model and landslide days as the driver for a landslide model. In a case study of the Panama Canal Watershed, this model describes spatial and temporal patterns of annual yields with a high degree of efficacy, demonstrating that simple daily data can be used to determine whether a river basin, such as the upper Río Charges basin, might be undergoing substantial landslide-related erosion.

In situ-produced cosmogenic ¹⁰Be was measured in 17 sediment samples to estimate the rate and distribution of sediment generation in the upper Río Chagres basin over the last 10 to 20 kyr. Results indicate that the upper Río Chagres basin is generating sediment uniformly. Nuclide activities suggest basin-wide sediment generation rates of 143 and 354 tons/km/yr (avg. = 234 ± 74 tons/km/yr; n = 7) for small tributary basins and 248 to 281 tons/km/yr (avg. = 267 ± 97 tons/km/yr; n = 3) for large tributary basins. The weighted average of all tributaries is 269 ± 63 tons/km/yr; n = 10). A sample collected upstream of Lago Alhajuela suggests that the entire basin is exporting sediment at a rate of 275 ± 62 tons/km/yr. These cosmogenic nuclide measurements all suggest that the upper Río Chagres basin (when considered on scales <5 km² to >350 km²) is generating sediment at ∼270 tons/km/yr. This long-term (1 −20 kyr) sediment generation rate that is equivalent to the estimate derived from suspended sediment yield measured below the upper Río Chagres- Rió Chico confluence from 1981–96 (289 tons tons/km/yr). Such similarity implies that decadal and millennial sediment yields are similar. Thus, short-term sediment yields and long-term sediment generations are in balance, implying steady landscape behavior over time. The background sediment yield suggests that it would take ∼3,600 years to completely fill Lago Alhajuela, the reservoir for the Panama Canal. Taking into account the present day 2- to 3- fold increase in sediment yields for adjacent human-impacted Rió Boquerón and Rió Pequení basins, the filling time is reduced to ∼2,000 years. However, it would only take between 250 to 600 years to reduce the reservoir capacity (69% of maximum) enough to drain the entire reservoir for precipitation conditions similar to the 1982 El Niño event. Such models highlight the importance of proper watershed management in order to reduce the sedimentation of Lago Alhajuela.

An integrated hydrometeorological system was designed for the utilization of data from various sensors in the 3300 km² Panama Canal Watershed for the purpose of producing real-time, spatially distributed, mean areal rainfall estimates, and rainfall and flow forecasts. These forecasts are used by the Panama Canal Authority for water management. The system ingests raw data from a 10 cm weather radar, automated rain gauges and surface meteorological stations, streamflow gauges, radiosonde upper air profilers, and analysis and forecast fields from the operational 80 km Eta numerical weather prediction model of the US National Weather Service State estimators utilize all available data for cloud, soil and channel model state updating and rainfall and flow forecast variance generation. Merged radar and rain gage fields are produced in real time and are used to compute mean areal precipitation for each sub-catchment in the watershed. Results of real time operation for the years 2000 and 2001 show useful system forecasts during severe storm periods.

Human occupation of the Panama Canal Watershed has affected its land cover for the past century. A rule-based model was developed and applied to estimate changes in land use and subsequent carbon emissions over the next twenty years in the Eastern Panama Canal Watershed (EPCW). Projections show that the highest percent change in land use for the ‘new-road’ scenario compared to the ‘business-as-usual’ scenario is for urban areas, and the greatest cause of carbon emission is from deforestation. Thus, the most effective way to reduce carbon emissions to the atmosphere in the EPCW is by reducing deforestation. In addition to affecting carbon emissions, reducing deforestation would also protect the soil and water resources of the EPCW, which has important implications for the long-term sustainable operation of the Panama Canal.