hydrologic characteristics

Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment

Phillips C.B., Scatena F.N. Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment. Earth Surface Processes and Landforms. 2012. DOI: 10.1002/esp.3345

Urbanization through the addition of impervious cover can alter catchment hydrology, often resulting in increased peak flows during floods. This phenomenon and the resulting impact on stream channel morphology is well documented in temperate climatic regions, but not well documented in the humid tropics where urbanization is rapidly occurring. This study investigates the long-term effects of urbanization on channel morphology in the humid sub-tropical region of Puerto Rico, an area characterized by frequent high-magnitude flows, and steep coarse-grained rivers. Grain size, low-flow channel roughness, and the hydraulic geometry of streams across a land-use gradient that ranges from pristine forest to high density urbanized catchments are compared. In areas that have been urbanized for several decades changes in channel features were measurable, but were smaller than those reported for comparable temperate streams. Decades of development has resulted in increased fine sediment and anthropogenic debris in urbanized catchments. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. At-a-station hydraulic geometry shows that velocity makes up a larger component of discharge for rural channels, while depth contributes a larger component of discharge in urban catchments. The average bank-full cross-sectional area of urbanized reaches was 1.5 times larger than comparable forested reaches, and less than the world average increase of 2.5. On average, stream width at bank-full height did not change with urbanization while the world average increase is 1.5 times. Overall, this study indicates that the morphologic changes that occur in response to urban runoff are less in channels that are already subject to frequent large magnitude storms. Furthermore, this study suggests that developing regions in the humid tropics shouldn’t rely on temperate analogues to determine the magnitude of impact of urbanization on stream morphology. Copyright © 2012 John Wiley & Sons, Ltd.

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

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.

C and N dynamics in the riparian and hyporheic zones of a tropical stream, Luquillo Mountains, Puerto Rico

C and N Dynamics in the Riparian and Hyporheic Zones of a Tropical Stream, Luquillo Mountains, Puerto Rico
Tamara J. Chestnut and William H. McDowell
Journal of the North American Benthological Society
Vol. 19, No. 2 (Jun., 2000), pp. 199-214

Hydrologic and chemical characteristics were determined for both riparian and hyporheic subsurface flow along a 100-m reach of a sandy-bottom tributary of the Rio Icacos in the Luquillo Experimental Forest, Puerto Rico. Hydrologic data (vertical hydraulic gradient and hydraulic conductivity of streambed sediments) and the topographic and morphological features of the watershed indicated diffuse inputs of groundwater from the near-stream riparian zone along this site. Cumulative groundwater discharge, determined by tracer dilution techniques, was ∼1.5 L/s or 10% of the total stream discharge. Spatial heterogeneity in hydrologic and chemical properties of riparian and hyporheic sediments was large. Hydraulic conductivity explained much of the variation in NH<sub>4</sub>-N and dissolved organic carbon (DOC) concentrations, with highest concentrations in sites having low conductivity. A mass-balance approach was used to examine the influence of the near-stream zone on nutrient transport and retention. Outwelling riparian groundwater had the potential to increase stream N concentrations by up to 84% and DOC concentrations by up to 38% along our 100-m reach. Because stream concentrations were constant downstream despite this input, we conclude that significant N and C retention or loss were occurring in the near-stream zone. Lotic ecosystems and their associated riparian groundwater can have a quantitatively significant impact on the nutrient budgets of tropical headwater catchments.
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