Geomorphology

Longitudinal Patterns in Stream Channel Geomorphology and Aquitic Habitat in the Luquillo Mountains of Puerto Rico

Abstract: 
The hydrologic, geomorphic, and ecological dynamics of tropical montane streams are poorly understood in comparison to many temperate and/or alluvial rivers. Yet as the threat to tropical freshwater environments increases, information on the dynamics of relatively pristine streams is important for understanding landscape evolution, managing and conserving natural resources, and implementing stream restoration. This dissertation characterizes the geomorphology and hydrology of five adjacent watersheds draining the Luquillo Experimental Forest (LEF) in northeastern Puerto Rico, and discusses implications on aquatic habitat. I performed several interrelated studies, including: 1) formulating a geographic information systems (GIS) framework to estimate hydrologic parameters from topographic information and hydrologic records, 2) developing a method to determine active stream channel boundaries (“bankfull” stage) that allows for comparison of channel geometry on the basis of flow-frequency, 3) decoupling the relative influences of lithologic and hydraulic controls on channel morphology using an extensive field-based stream survey and analysis of stream profiles, channel geometry, and sediment dynamics, 4) linking network- and pool-scale geofluvial dynamics to the abundance of migratory fish and shrimp through a collaborative analysis combining geomorphic surveys and aquatic faunal sampling. This research indicates that these streams have some properties resembling both temperate montane and alluvial rivers. Similar to low-gradient rivers where floodplains mark channel boundaries, the active channel stage in these streams is defined by the incipient presence of woody vegetation and soil development. Systematic basin-scale geomorphic patterns are well-developed despite apparent non-fluvial and lithologic control on local channel morphology. This implies that strong fluvial forces are sufficient to override channel boundary resistance; a feature common in self-forming “threshold” alluvial channels. Migratory aquatic fauna abundances are influenced by a variety of geomorphic factors such as barrier waterfalls and suitable headwater habitat, and are consequently highly variable and patchy. These results stand in contrast to the notion that aquatic communities mirror systematic geomorphic gradients, but rather acknowledges the influences of multiscale geomorphic processes. Ultimately, this research provides baseline information on physical and biological processes in relatively unaltered tropical streams and can be used to inform further studies that document human interactions with stream networks.

Streams of the Montane Humid Tropics. Treatise on Geomorphology

Scatena F.N., Gupta A., 2011. Streams of the Montane Humid Tropics. Treatise on Geomorphology. Editors E. Wohl. Academic Press, San Diego Ca. Vol 9. in press April 2011

Abstract: 
Tropical montane streams produce a disproportionately large amount of the sediment and carbon that reaches coastal regions and have often been considered to be distinct fluvial systems. They typically drain orogenic terrains that have not been recently glaciated, but have undergone climatic changes throughout the Pleistocene and currently receive 2000–3000 mm or more of precipitation each year. Steep gradient reaches with numerous boulders, rapids, and waterfalls that alternate with lower gradient reaches flowing over weathered rock or a thin veneer of coarse alluvium characterize these streams. Although their morphology and hydrology have distinctive characteristics, they do not appear to have diagnostic landforms that can be solely attributed to their low-latitude locations. While they are relatively understudied, an emerging view is that their distinctiveness results from a combination of high rates of chemical and physical weathering and a high frequency of significant geomorphic events rather than the absolute magnitudes of individual floods or other geomorphic processes. Their bedrock reaches and abundance of large and relatively immobile boulders combined with their ability to transport finer-grained sediment also suggest that the restorative processes in these systems may be less responsive than in other fluvial systems.

Twelve testable hypotheses on the Geobiology of weathering

Brantley S.L., Megonigal J.P., Scatena F.N. et al 2010. Twelve testable hypotheses on the Geobiology of weathering. Geobiology. DOI: 10.1111/j.1472-4669.2010.00264.x

Abstract: 
Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth’s surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term.(4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes.(5) Biology shapes the topography of the Critical Zone.(6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws.(7) Rising global temperatures will increase carbon losses from the Critical Zone.(8) Rising atmospheric PCO2 will increase rates and extents of mineral weathering in soils.(9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering.(10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales.(12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

Sediment production from unpaved roads in a sub-tropical dry setting — Southwestern Puerto Rico

Carlos E. Ramos Scharrón
Sediment production from unpaved roads in a sub-tropical dry setting — Southwestern Puerto Rico
CATENA
Volume 82, Issue 3, 15 September 2010, Pages 146-158

Abstract: 
The threat imposed by increased sediment loading rates ranks among the most important stressors affecting coral reef ecosystems worldwide. This study represents an effort to quantify the effects of unpaved roads on erosion rates in a dry sub-tropical area of Puerto Rico and is intended to aid in developing scientifically-based erosion mitigation strategies. Hence, the specific objectives of this study were to: (1) measure sediment production rates from unpaved roads; (2) evaluate the effect of precipitation, rainfall erosivity, slope, plot length, and vegetation cover on sediment production rates; and (3) compare measured sediment production rates to published surface erosion data from roaded and natural sites in the Eastern Caribbean. Sediment production from nine abandoned road segments with varying slopes and plot lengths were measured with sediment traps in southwestern Puerto Rico from August 2003 to September 2005. The overall average sediment production rate was 0.84 Mg ha−1 yr−1, and the range of observed values was 15–50 times higher than locally-measured natural erosion rates. Only four of the nine study sites had a statistically significant correlation between sediment production and total rainfall and this is attributed to progressive changes in some of the conditions controlling erosion rates. Sediment production rates were dependent on slope raised to the 1.6th power, as well as to the product of plot length times slope1.6. Average erosion rates were inversely but poorly related to vegetation cover. An observed decline in sediment production rates was observed for all nine study segments, and this amounted to a statistically significant difference between observations made during the early stages of monitoring (Period 1: August 2003–April 2004) relative to those during the latter parts of the study (Period 2: May 2004– September 2005). Annual erosion rates during Period 1 amounted to 0.18 to 4.0 Mg ha−1 yr−1 for road segments with 1% and 22% slopes, respectively; rates during Period 2 were between 0.024 and 0.52Mg ha−1 yr−1, or only 13% of those during Period 1. Differences in sediment production rates between the two periods are attributed to more intense rainfall during Period 1 and to a notably higher vegetation cover during Period 2. Rainfall appears to play a paradoxical role in controlling surface erosion rates on abandoned road surfaces in a sub-tropical dry region.While ample rainfall is needed to generate erosion by rainsplash and overland flow, once rainfall satisfies soil moisture requirements for sustaining vegetation colonization it may also contribute to declining sediment production rates. Therefore, any model that attempts to properly address the temporal variation in erosion rates occurring on abandoned roads in a climatic setting where moisture availability is a limiting factor must not only follow the more traditional surface armoring-based approach but must also integrate the effects of re-vegetation. Such types of modelswill eventually become useful tools to properly assess the effects of past, current, and future land use practices on erosion rates, and to improve mitigation and land development strategies to lessen the impact on vital marine habitats

Bermeja Complex Serpentinites: Testing Theories of Emplacement

Abstract: 
The Bermeja Complex in southwestern Puerto Rico contains large blocks of serpentinite of uncertain origin. Competing theories have emerged to explain the presence of these rocks. One theory is that the serpentinite represents the serpentinized peridotite section of an obducted seafloor ophiolite. An alternative theory states that the rocks are part of a volcanic island arc terrane in which large blocks of serpentinite from a subducted slab were squeezed up through the crust and solidified within the Puerto Rico crust. We have studied the mineralogy, chemical composition, and outcrop-scale geology of our field site in the southernmost belt of the Bermeja Complex, the Sierra Bermeja mountains. Our main goal was to sample the serpentinites and test the concentration of trace elements and rare earth elements to infer the origin of the complex. Our trace element analysis was used to provide constraints on the origin of the serpentinite, being either crust or mantle derived. The combination of mineralogical analysis, bulk chemical analysis, outcrop-scale measurements, and trace element concentration data have provided sufficient evidence to resolve the issue of the Sierra Bermeja serpentinite origin. We have determined that the serpentinites of our field site were emplaced by the obduction of an ophiolite sequence. Our results have implications for the complex tectonic history of the PRVI microplate and the Caribbean region as a whole.

Some Tropical Landforms of Puerto Rico

Monroe, W. H. 1980. Some tropical land forms of
Puerto Rico. Geological Survey Professional Paper
1159. US Governmnet Printing Office, Washington,
D.C.

Abstract: 
part2

Some Tropical Landforms of Puerto Rico

Monroe, W. H. 1980. Some tropical land forms of
Puerto Rico. Geological Survey Professional Paper
1159. US Governmnet Printing Office, Washington,
D.C.

Depositional History and Evolution of the Paso del Indio Site, Vega Baja, Puerto Rico

Jeffrey J. Clark, Jeff Walker, and Reniel Rodrı´guez Ramos
Depositional History and Evolution of the Paso del Indio Site, Vega Baja, Puerto Rico
Geoarchaeology An International Journal (2003)
Volume: 18, Issue: 6, Pages: 625-648

DOI: 10.1002/gea.10083

Abstract: 
Potshards discovered during excavation of bridge pilasters for a major expressway over the Rio Indio floodplain, a stream incised within the karsts of north-central Puerto Rico, required large-scale archaeological excavation. Five-meter-deep bridge pilaster excavations in the alluvial valley provide a 4500-year history of deposition. Stratigraphic analysis of the exposed pilaster walls in combination with textural and organic carbon analyses of sediment cores obtained over a much broader area suggest a fluvial system dominated by overbank deposition. Six sequences of alternating light and dark layers of sediment were identified. The darker layers are largely composed of silts and clays, whereas the lighter layers are rich in sandsized sediment. Archaeological evidence indicates the organic-rich dark layers, believed to be buried A horizons, coincide with pre-historic occupation by Cedrosan Saladoid, Elenan Ostionoid, and Chican Ostionoid, extending from A.D. 450 to A.D. 1500. Lighter layers below the dark soil horizons are interpreted as overbank deposits from large magnitude flood events. The floodplain aggraded discontinuously with rapid deposition of sand followed by gradual accumulation of silt, clay, and organic material. An approximately 1-m-thick layer of coarse sand and gravel halfway up the stratigraphic column represents an episode of more frequent and severe floods. Based on radiocarbon ages, this layer aggraded between A.D. 1000 and A.D. 1100, which is well within the Elenan Ostionoid era (A.D. 900–1200). Rates of sedimentation during this period were approximately 8 mm per year, ten times greater than the estimates of sedimentation rates before and after this flood sequence. The cause for the change in deposition is unknown. Nonetheless the Elenan Ostionoid would have had to endure frequent loss of habitation structures and crops during these events.

Effects of land use change on northeastern Puerto Rican rivers

Clark, J. 1997. Effects of land use change on northeastern
Puerto Rican rivers. Phd Dissertation, Johns
Hopkins University, Baltimore, Maryland, 187
pages.

Abstract: 
The 500 year period considered in this study is sufficiently long that river change may be produced by natural changes in the external controls of river geometry. These controls include changes in the base level and changes in the climate which, like land use, change the supply of sediment to the rivers.

Effects of land-use change on channel morphology in northeastern Puerto Rico

Clark, J. J., and P. R. Wilcock (2000), Effects of land-use change on
channel morphology in northeastern Puerto Rico, Geol. Soc. Am. Bull.,
112(12), 1763– 1777.

Abstract: 
Between 1830 and 1950 much of northeastern Puerto Rico was cleared for agriculture. Runoff increased by 50% and sediment supply to the river channels increased by more than an order of magnitude. Much of the land clearance extended to steep valley slopes, resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels. A shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased, allowing the rivers to begin removing coarse sediment stored within their channels. The size, abundance, and stratigraphic elevation of in-channel gravel bar deposits increases, channel depth decreases, and the frequency of overbank flooding increases downstream along these channels. This is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment. A downstream decrease in channel size is contrary to the expected geometry of self-adjusted channels, but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period. Reverse (downstream decreasing) channel morphology is not often cited in the literature, although consistent observations are available from areas with similar land-use history. Identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent, but scattered downstream trend. Identification of reverse channel morphology along individual streams in areas with similar land-use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments.
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