Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico

Pike, Andrew S.; Scatena, F.N.; Wohl, Ellen E. 2010. Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico. Earth Surface Processes and Landforms. DOI: 10.1002/esp.1978.

An extensive survey and topographic analysis of fi ve watersheds draining the Luquillo Mountains in north-eastern Puerto Rico was conducted to decouple the relative infl uences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic infl uence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS-based topographic analysis was used to examine channel profi les, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profi les are generally well graded but have concavities that refl ect the infl uence of multiple rock types and colluvial-alluvial transitions. Non-fl uvial processes, such as landslides, deliver coarse boulder-sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fi ning in the downstream reaches; a pattern associated with a mid-basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid-basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull fl ow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self-forming ‘threshold’ alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach-scale channel morphology, strong fluvial forces acting over time have been suffi cient to override boundary resistance and give rise to systematic basin-scale patterns.

Riparian indicators of flow frequency in a tropical montane stream network

Pike, A. S., and Scatena, F. N., 2010, Riparian indicators of flow frequency in a tropical montane stream
network: Journal of Hydrology, v. 382, p. 72–87, doi:10.1016/j.jhydrol.2009.12.019.

Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to no work has been done to establish such relationships between field features and the flow regime in these environments. Furthermore, the transition between the active channel of a river and the adjacent flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain stream network and (b) identifies a reference active channel boundary in these channels, based on statistically defined combinations of riparian features, that corresponds to the same flow frequency of the bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge, flow frequency, and recurrence intervals associated with these features was then determined from longterm 15-min discharge records and a partial duration series analysis. Survey data indicate that mosses and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is associated with intermediate discharges that correspond to the threshold for sediment mobilization. Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a less frequent discharge that is coincident with the effective discharge of bed load sediment transport. Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features in steepland sites has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies influence the establishment of riparian vegetation, soil development, and substrate characteristics along channel margins in similar ways. Thus, these riparian features can be used as an indicator of hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency throughout the study stream network.

The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico

LÓPEZ, TANIA DEL MAR; AIDE, T. MITCHELL; SCATENA F. N. 1998. The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico. Caribbean Journal of Science, Vol. 34, No. 3-4, 298-307, 1998.

The Revised Universal Soil Loss Equation (RUSLE) was used in conjunction with a Geographic Information System to determine the influence of land use and other environmental factors on soil erosion in the Guadiana watershed in Puerto Rico. Mean annual erosion, suspended sediment discharge, and the rainfall-erosion factor of the RUSLE increased with annual rainfall. Median soil erosion rates varied among the seven land uses: bare soil (534 Mg ha-1 yr-1), open canopy forest (26 Mg ha-1 yr-1), agriculture (22 Mgha-1 yr -1), pasture (17 Mg ha -1 yr -1), less dense urban (15 Mg ha-1 yr -1), closed canopy forest (7 Mg ha -1 yr -1), and dense urban (1 Mg ha-1 yr -1). The differences between open canopy forest, agriculture, pasture, and less dense urban were not significantly different but median values for open canopy forests were slightly greater because they occurred on steep slopes. The five-year average sediment delivery ratio for the basin was 0.17, which is comparable to delivery ratios estimated for watersheds of similar size. Simulations of different land use configurations indicate that reforestation of 5% of the watershed with the highest erosion rates would decrease basin wide erosion by 20%. If the entire watershed was reforested, soil erosion would be reduced by 37%.

Emergy Evaluation of Reforestation Alternative in Puerto Rico

Odum, H.T., Doherty, S.J., Scatena, F.N., Kharecha, P.A., 2000. Emergy
evaluation of reforestation alternatives in Puerto Rico. Forest Science
46 (4), 521–530.

Six alternative ways of reforesting degraded lands in Puerto Rico were evaluated using emergy (spelled with an “m”). Emergy and its economic equivalent, emdollars, put the contributions of environmental work and human services on a comparable basis. This article shows the emergy method for evaluating forest contributions to public benefit and its use to select alternatives for reforestation. Emdollar values were compared for six scenarios for reforestation of degraded land in Puerto Rico: (1) the natural succession within or adjacent to mature forest; (2) reforestation from the spread of the exotic tree siris (Albizia lebbek); (3) reforestation with plantations of siris and mahogany for harvest; (4) reforestation by leaving plantations unharvested; (5) direct planting of seedlings of many species; and (6) starting patches of forest by massive transfer of topsoil, seed bank, and roots. After energy systems diagrams were made for each reforestation alternative, data were assembled and evaluation tables prepared that estimated the emergy required for: (1) canopy closure and (2) developing species complexity if left unharvested. To explain the method, detailed calculations were included for one of the alternatives, exotic Albizia lebbek plantation on 11 yr harvest cycle. All alternatives generated net public benefit (emdollar yield ratios 4.2 to 24.3). The emdollar value of a closed canopy developed in 10 to 20 yr ranged from 20,000 to 48,000 em$ /ha, whereas the economic costs were $1200 to $9700. For complex forest development in 25 to 60 yr, values ranged from 63,000 to 118,000 em$ /ha, much higher than economic costs of $4000 to $12,000/ha. Highest public benefit per dollar cost came from succession (24.7 em$/$) and exotic colonization (19.1 em$/$). Highest potential monetary returns were from exotic spread (15.1 $/$) and plantations (17.9 and 14.5 $/$). Stand quality after 60 yr, as measured by the transformity (emergy/energy), was largest in mahogany plantation (6.4 × 10 4 sej/J) and succession forest (3.9 × 104 sej/J).

An EMERGY Evaluation of Puerto Rico and the Luquillo Experimental Forest

Scatena, F.N.; Doherty, S.J.; Odum, H.T.; Kharecha, P. 2002. An EMERGY
evaluation of Puerto Rico and the Luquillo Experimental Forest. Gen. Tech. Rep.
IITF-GTR-9. Río Piedras, PR: U.S. Department of Agriculture, Forest Service,
International Institute of Tropical Forestry. 79 p.

The many functions of Puerto Rico and the Luquillo Experimental Forest (the Forest) were evaluated in units of solar EMERGY, an energy-based measure of resource contribution and influence, defined as the energy of one type required to produce a flow or storage of another type. Rainfall and tectonic uplift are the largest environmental inputs into the Forest. The interaction of these inputs results in an erosional landscape where the EMERGY of biological processes is less than the EMERGY associated with the physical and chemical sculpturing of the landscape. The environmental work that built the natural capital of these forests is 9 to 50 times their current dollar market values. Of the investments evaluated in this study, the effects associated with water extraction are the largest. Tectonic inputs and the hydrologic cycle also provide most of the environmental EMERGY flows in the island of Puerto Rico. The ratio of societal inputs to environmental inputs, however, is 45 for Puerto Rico and 3.5 for the Forest. Per capita EMERGY- use is typical of moderately developed economies, but the island has one of the most investment-intensive, least self-sufficient economies known and an EMERGY signature that resembles a city-state.

Vertical motions of the Puerto Rico Trench and Puerto Rico and their cause

ten Brink US (2005) Vertical motions in the Puerto Rico trench and
Puerto Rico and their cause. J Geophys Res 100:B06404. doi:

The Puerto Rico trench exhibits great water depth, an extremely low gravity anomaly, and a tilted carbonate platform between (reconstructed) elevations of +1300 m and 4000 m. I argue that these features are manifestations of large vertical movements of a segment of the Puerto Rico trench, its forearc, and the island of Puerto Rico that took place 3.3 m.y. ago over a time period as short as 14–40 kyr. I explain these vertical movements by a sudden increase in the slab’s descent angle that caused the trench to subside and the island to rise. The increased dip could have been caused by shearing or even by a complete tear of the descending North American slab, although the exact nature of this deformation is unknown. The rapid (14–40 kyr) and uniform tilt along a 250 km long section of the trench is compatible with scales of mantle flow and plate bending. The proposed shear zone or tear is inferred from seismic, morphological, and gravity observations to start at the trench at 64.5Wand trend southwestwardly toward eastern Puerto Rico. The tensile stresses necessary to deform or tear the slab could have been generated by increased curvature of the trench following a counterclockwise rotation of the upper plate and by the subduction of a large seamount.

Bivergent thrust wedges surrounding oceanic island arcs: Insight from observations and sandbox models of the northeastern Caribbean plate

ten Brink U, Marshak S, Granja JL (2009) Bivergent thrust wedges
surrounding oceanic island arcs: insights from observations and
sandbox models in the north-eastern Caribbean plate. Geol Soc
Am Bull 121:1522–1536

At several localities around the world, thrust belts have developed on both sides of oceanic island arcs (e.g., Java-Timor, Panama, Vanuatu, and the northeastern Caribbean). In these localities, the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt. For example, in the northeastern Caribbean, a north-verging accretionary prism lies to the north of the Eastern Greater Antilles arc (Hispaniola and Puerto Rico), whereas a south-verging thrust belt called the Muertos thrust belt lies to the south. Researchers have attributed such bivergent geometry to several processes, including: reversal of subduction polarity; subduction-driven mantle fl ow; stress transmission across the arc; gravitational spreading of the arc; and magmatic infl ation within the arc. New observations of deformational features in the Muertos thrust belt and of fault geometries produced in sandbox kinematic models, along with examination of published studies of island arcs, lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity, without subarc mantle fl ow, and without magmatic infl ation. We suggest that the Eastern Greater Antilles arc and comparable arcs are simply crustalscale bivergent (or “doubly vergent”) thrust wedges formed during unidirectional subduction. Sandbox kinematic modeling suggests, in addition, that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly. In such cases, the arc acts as a backstop that transmits compressive stress into the backarc region. Further, modeling shows that when arcs behave as rigid blocks, the strike-slip component of oblique convergence is accommodated entirely within the prowedge and the arc—the retrowedge hosts only dip-slip faulting (“frontal thrusting”). The existence of large retrowedges and the distribution of faulting in an island arc may, therefore, be evidence that the arc is relatively rigid. The rigidity of an island arc may arise from its mafi c composition and has implications for seismic-hazard analysis.

Measurement and prediction of natural and anthropogenic sediment sources, St. John, U.S. Virgin Islands

Ramos- Scharrón, C.E., and L.H. MacDonald. 2007.
Measurement and prediction of natural and
anthropogenic sediment sources, St. John, U.S.
Virgin Island. Catena. 71: 250-266.

A quantitative understanding of both natural and anthropogenic sediment sources is needed to accurately assess and predict the potentially adverse effects of land development on aquatic ecosystems. The main objective of this study was to quantify sediment production and delivery rates in a dry tropical environment on the island of St. John in the eastern Caribbean. One to three years of measurements were used to determine values and empirical functions for estimating sediment production from streambanks, treethrow, undisturbed hillslopes, zero-order subcatchments, unpaved road surfaces, and road cutslopes. Sediment production also was measured from both undisturbed and roaded first-order subcatchments. Among natural sources of sediment, streambanks had the highest mean erosion rate at 100 Mg ha−1 yr−1. The uprooting of trees along stream margins is estimated to generate approximately of 0.2 Mg of sediment per kilometer of stream per year, or about 0.1 Mg ha−1 yr−1 for a stream corridor that consists of a 9-m wide channel and a 3-m wide buffer zone. Undisturbed 40 m2 hillslope plots generated 0.01 to 0.27 Mg ha−1 yr−1. Mean sediment yields from undisturbed zero- and first-order catchments were only 0.01 and 0.08 Mg ha−1 yr−1, respectively. Unpaved roads that were graded at least every other year had sediment production rates ranging from 57 Mg ha−1 yr−1 for a road with a 2% slope to 580 Mg ha−1 yr−1 for a road with a 21% slope. Sediment production rates from ungraded roads were about 40% lower than those from recently graded roads, while production rates from steep abandoned roads were only 12 Mg ha−1 yr−1. Cutslope sediment production rates ranged from 20 to 170 Mg ha−1 yr−1, but their contribution to sediment yields at the road segment scale was relatively small. Since unpaved roads increase hillslope-scale sediment production rates by several orders of magnitude, the first-order catchments with unpaved roads had sediment yields that were at least five times higher than undisturbed catchments. The relative importance of each sediment source varies from catchment to catchment as a result of the abundance and spatial distribution of landscape types. The values and predictive functions developed in this study have been incorporated into a GIS-based model to predict catchmentscale sediment yields. Application of this model to three basins in St. John suggest that unpaved roads are currently the dominant sediment source, and that they are responsible for increasing watershed-scale sediment yields by 3–9 times relative to undisturbed conditions. Both the data from the present study and the GIS model can help estimate sediment production and catchment-scale sediment yields in similar environments. © 2007 Elsevier B.V. All rights reserved.

Landsliding and Its Multiscale Influence on Mountainscapes

Restrepo, Carla; Walker, Lawrence R.; Shiels, Aaron B.; Bussmann, Rainer; Claessens, Lieven; Fisch, Simey; Lozano, Pablo; Negi, Girish; Paolini, Leonardo; Poveda, Germán; Ramos-Sharrón, Carlos; Ritcher, Michael; Velázquez, Eduardo. 2009. Landsliding and its multiscale influence on mountainscapes. Bioscience. 59(8): 685-698.

Landsliding is a complex process that modifies mountainscapes worldwide. Its severe and sometimes long-lasting negative effects contrast with the less-documented positive effects on ecosystems, raising numerous questions about the dual role of landsliding, the feedbacks between biotic and geomorphic processes, and, ultimately, the ecological and evolutionary responses of organisms. We present a conceptual model in which feedbacks between biotic and geomorphic processes, landslides, and ecosystem attributes are hypothesized to drive the dynamics of mountain ecosystems at multiple scales. This model is used to integrate and synthesize a rich, but fragmented, body of literature generated in different disciplines, and to highlight the need for profitable collaborations between biologists and geoscientists. Such efforts should help identify attributes that contribute to the resilience of mountain ecosystems, and also should help in conservation, restoration, and hazard assessment. Given the sensitivity of mountains to land-use and global climate change, these endeavors are both relevant and timely.

Caribbean Geology: An Introduction

DONOVAN, S. K., AND T. A. JACKSON. 1994. Caribbean
Geology: An Introduction. Univ. of the West Indies
Publishers' Assn., Kingston, Jamaica.

THE LITERATURE of the geology of the Caribbean region is widely dispersed through a number of primary sources. Apart from numerous research papers in international and regional scientific journals, there are also the transactions that have arisen from the various Caribbean, Latin American and Central American Geological Conferences (for references, see Draper and Dengo), plus various smaller, often more specialized meetings, particularly in Jamaica and Trinidad. To this burgeoning list can be added various review volumes, newsletters and unpublished reports. Various general works have been published which review this enormous literature, the most recent examples being edited by Nairn and Stehli (now almost 20 years old) and Dengo and Case1. These volumes are generally excellent, but are intended mainly as specialist references and are generally too expensive, and often too advanced or specialized, for student readers. This is particularly unfortunate for any undergraduate taking an advanced course in Caribbean geology or, for that matter, any new graduate student starting research in the region. Caribbean Geology: An Introduction has been produced to help fill the need for a cheap, but comprehensive, text on Caribbean geology. The 15 chapters have been written by the editors and a group of invited authors who are experts in particular aspects of the geology of the region. While we have attempted to be as comprehensive as possible, it is hoped that the text is pitched at a level that is both intelligible and informative to the student as well as the expert.
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