Geomorphology

Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico

Covich, Alan P.; Crowl, Todd A.; Heartsill-Scalley, Tamara 2006. Effects of drought and hurricane disturbances on headwater distributions of palaemonid river shrimp (Macrobrachium spp.) in the Luquillo Mountains, Puerto Rico.. J. N. Am. Benthol. Soc., 25(1):99-107.

Abstract: 
Extreme events (hurricanes, floods, and droughts) can influence upstream migration of macroinvertebrates and wash out benthic communities, thereby locally altering food webs and species interactions. We sampled palaemonid river shrimp (Macrobrachium spp.), dominant consumers in headwaters of the Luquillo Mountains of northeastern Puerto Rico, to determine their distributions along an elevational gradient (274–456 m asl) during a series of disturbances (Hurricane Hugo in 1989, a drought in 1994, and Hurricane Georges in 1998) that occurred over a 15-y period (19882002).We measured shrimp abundance 3 to 6 times/y in Quebrada Prieta in the Espiritu Santo drainage as part of the Luquillo Long-Term Ecological Research Program. In general, Macrobrachium abundance declined with elevation during most years. The lowest mean abundance of Macrobrachium occurred during the 1994 drought, the driest year in 28 y of record in the Espiritu Santo drainage. Macrobrachium increased in abundance for 6 y following the 1994 drought. In contrast, hurricanes and storm flows had relatively little effect on Macrobrachium abundance.

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.

THE EFFECTS OF BIOTURBATION ON SOIL PROCESSES AND SEDIMENT TRANSPORT

Emmanual J Gabet, O J Reichman, and Eric W Seabloom (2003)
The Effects of Bioturbation on Soil Processes and Sediment Transport
Annual Review Earth Planet Science:249-73.

Abstract: 
Plants and animals exploit the soil for food and shelter and, in the process, affect it in many different ways. For example, uprooted trees may break up bedrock, transport soil downslope, increase the heterogeneity of soil respiration rates, and inhibit soil horizonation. In this contribution, we review previously published papers that provide insights into the process of bioturbation. We focus particularly on studies that allowus to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production. Using geometrical relationships and data from others’ work, we derive simple sediment flux equations for tree throw and root growth and decay.

Exploring Potential Spatial-Temporal Links Between Fluvial Geomorphology and Nutrient-Periphyton Dynamics in Streams Using Simulation Models

Doyle, MW, Stanley EH. 2006. Exploring potential spatial-temporal links between fluvial geomorphology and nutrient-periphyton dynamics in streams using simulation models. Annals of the Association of American Geographers. 96:687-698.

Abstract: 
Understanding stream ecosystem processes necessitates an awareness of not only the physical, chemical, and biological systems, but also how these separate systems interact with each other. Using a nutrient retention and periphyton growth model coupled to a dynamic geomorphic template, we explore the potential role of channel shape, slope, and sediment texture on downstream nutrient retention, and attempt to determine if physical changes alone can drive or influence changes in nutrient-periphyton dynamics. The overall model results suggest there is a strong potential control of both nutrient retention and periphyton biomass by channel morphology. For example, with constant biochemical process rates, geomorphic variations alone could alter spatial distribution of nutrient retention over a 4-km study reach by between 11 percent and 52 percent. These results suggest channel geomorphology has a potentially strong influence on both nutrient retention and basal food sources in streams. Key Words: biogeochemistry, ecohydrology, ecosystem ecology, river restoration.

Infiltration on mountain slopes: a comparison of three environments

Harden,Carol P.; Scruggs, P. Delmas 2003. Infiltration on mountain slopes: a comparison of three environments.. Geomorphology 55 ;5 -24.

Abstract: 
Water is well established as a major driver of the geomorphic change that eventually reduces mountains to lower relief landscapes. Nonetheless, within the altitudinal limits of continuous vegetation in humid climates, water is also an essential factor in slope stability. In this paper, we present results from field experiments to determine infiltration rates at forested sites in the Andes Mountains (Ecuador), the southern Appalachian Mountains (USA), and the Luquillo Mountains (Puerto Rico). Using a portable rainfall simulator–infiltrometer (all three areas), and a single ring infiltrometer (Andes), we determined infiltration rates, even on steep slopes. Based on these results, we examine the spatial variability of infiltration, the relationship of rainfall runoff and infiltration to landscape position, the influence of vegetation on infiltration rates on slopes, and the implications of this research for better understanding erosional processes and landscape change. Infiltration rates ranged from 6 to 206 mm/h on lower slopes of the Andes, 16 to 117 mm/h in the southern Appalachians, and 0 to 106 mm/h in the Luquillo Mountains. These rates exceed those of most natural rain events, confirming that surface runoff is rare in montane forests with deep soil/regolith mantles. On well-drained forested slopes and ridges, apparent steadystate infiltration may be controlled by the near-surface downslope movement of infiltrated water rather than by characteristics of the full vertical soil profile. With only two exceptions, the local variability of infiltration rates at the scale of 10j m overpowered other expected spatial relationships between infiltration, vegetation type, slope position, and soil factors. One exception was the significant difference between infiltration rates on alluvial versus upland soils in the Andean study area. The other exception was the significant difference between infiltration rates in topographic coves compared to other slope positions in the tabonuco forest of one watershed in the Luquillo Mountains. Our research provides additional evidence of the ability of forests and forest soils to preserve geomorphic features from denudation by surface erosion, documents the importance of subsurface flow in mountain forests, and supports the need for caution in extrapolating infiltration rates.
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