United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

The search for organic substances and inorganic volatile compounds in the surface of Mars

Abstract: A total of four Martian samples, one surface and one subsurface sample at each of the two Viking landing sites, Chryse Planitia and Utopia Planitia, have been analyzed for organic compounds by a gas chromatograph-mass spectrometer. In none of these experiments could organic material of Martian origin be detected at detection limits generally of the order of parts per billion and for a few substances closer to parts per million. The evolution of water and carbon dioxide, but not of other inorganic gases, was observed upon heating the sample to temperatures of up to 500 C. The absence of organic compounds seems to preclude their production on the planet at rates that exceed the rate of their destruction. It also makes it unlikely that living systems that behave in a manner similar to terrestrial biota exist, at least at the two Viking landing sites.

The role of refugia for fishes during drought: a review and synthesis

Abstract: SUMMARY 1. Drought is a natural disturbance of aquatic ecosystems and can be a major factor in structuring aquatic communities. For individuals, populations and communities to persist in disturbed environments, they must have refuge from disturbance or disturbance must be minimal. Refugia convey spatial and temporal resistance or resilience in the face of disturbance, but the role of refugia in aquatic systems remains poorly understood. 2. We review available literature on aquatic refugia for fishes in order to synthesise current knowledge and provide suggestions for needed research. Our objectives were to clarify definitions of disturbance and refugia in the context of drought in aquatic systems, review how refuge habitats influence fish community structure, and consider the potential impact of refugia on fish population and community dynamics during drought. 3. Droughts cause a decrease in surface area/volume and an increase in extremes of physical and chemical water quality parameters. These conditions are linked with biotic interactions that structure the community of fishes residing in low-flow or dry season refugia by increasing mortality rates, decreasing birth rates and/or increasing migration rates. Many aquatic organisms seek refuge from disturbance and/or have adaptations (e.g. physiological tolerance) that provide refuge. 4. Drought in aquatic systems leads to shifts in refugia spacing and connectance at multiple spatial and temporal scales. Refuge size, disturbance intensity, and mobility of organisms is predicted to play a large role in population persistence. We expect that refuge habitats will experience net immigration during drying and net emigration after rewetting, with the opposite occurring in surrounding habitat patches. Population dynamics of fishes using refugia during drought are best modelled by modified source-sink dynamics, but dynamics are likely to change with spatial scale.

Sinuosity of Alluvial Rivers on the Great Plains

Abstract: Data on the morphologic and sediment characteristics of stable alluvial rivers of the Great Plains were collected at 50 cross sections. The channel patterns of these rivers were classified into five types: tortuous, irregular, regular, transitional, and straight. Because no clear demarcation existed between each of the types, the pattern of the rivers was described by sinuosity, a ratio of channel length to valley length. The sinuosity ( (P) ) of these rivers is related to the shape of the channels expressed as a width-depth ratio ( F ) and to the percentage of silt and clay in the perimeter of the channel ( M ) as follows: ![Formula][1] ![Formula][2] Sinuous streams are characterized by a low width-depth ratio ( F ), a high percentage of silt-clay in the perimeter of the channel ( M ), a high percentage of silt-clay in the banks (although the banks of straight channels may also contain large amounts of silt-clay), and a lower gradient than straight channels having the same mean discharge. Discharge itself does not appear to affect the sinuosity of streams. Another possible distinction between straight and sinuous streams is in the proportions of the components of total sediment load. In a wide, shallow channel much of the sediment transported is bed-material load. In a narrow, deep channel most of the sediment transported is wash load. On the Great Plains both straight and sinuous streams may flow on the surface of alluvial valley fills at about the same valley slope. The departure of a stream from a straight course down the alluvial valley results from changes in both the caliber of the sediment load and in the relative proportions of bed-material load and wash load during the post-Pleistocene alluviation of these valleys. When during this alluviation the proportion of wash load increased, most probably by a decrease in bed-material load, the stream adjusted itself by decreasing its gradient through the development of a sinuous course. Recent changes in stream sinuosity in response to changes in the proportions of bed load and suspended load support this hypothesis. [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif