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earth and environmental sciences

Bank Full Of It

Submitted by jdp on Wed, 08/26/2015 - 04:38 pm

Fluvial geomorphologists, along with hydrologists and river engineers, have long been concerned with the flows or discharges that are primarily responsible for forming and shaping river channels. In the mid-20th century it was suggested that this flow is associated with bankfull stage—the stage right at the threshold of overflowing the channel—and that this occurs, on average, about every year or two in humid-climate perennial streams. If you have to choose just one flow to fixate on—and sometimes you do, for various management, design, and assessment purposes—and have no other a priori information about the river, bankfull is indeed the best choice. But, of course, nature is not that simple.

Fluviodiversity

Submitted by jdp on Fri, 08/14/2015 - 05:32 pm

One of the classic principles/relationships in biogeography is called the species-area curve, relating the number of different species found (usually of some particular taxonomic group; e.g., birds or plants) to the area sampled. These curves are usually well fit by an exponential relationship:

S = c A b

where S is the number of species, A is area, c is a constant representing the number of species in the smallest area sampled, and b represents the rate of increase of species with area. While b could be greater than 1 if major biogeographical boundaries are transgressed (so that whole new sets of species are encountered), otherwise b < 1, and usually much less; 0.25 is a fairly common value.

Juanjo Ibanez and I (in separate studies) found that similar trends apply to soil diversity, with S in this case indicating number of different soil types (e.g., soil series). In his very broad scale analyses, Juanjo also found b » 0.25, while in my landscape-scale studies b was in the range of 0.6.  Syntheses of this work are found in the book Pedodiversity (CRC Press, 2013) edited by Ibanez and James Bockheim.

The Curious Expansion of Polly's Bend

Submitted by jdp on Thu, 08/06/2015 - 08:39 am

Though the meander bends in the Kentucky River gorge area are considered to be mostly inherited (i.e., they were there before the river began downcutting about 1.5 million years ago), they are not static features. This continues a previous post looking at Polly’s Bend.

Geologic map of Polly’s Bend (from Kentucky Geological Survey’s Geologic Mapping Service). Ollr, Oto, Ocn are all Ordovician limestones. Qal is Quaternary alluvium, and the stippled pattern with the red + is Quaternary fluvial terrace deposits. Polly’s Bend is about 5 km in maximum width.

Polly’s Bend: Initial Conditions

Submitted by jdp on Sun, 08/02/2015 - 08:46 am

South of Lexington and north of Danville, Kentucky, the Kentucky River makes a major turn from a generally SW to NW direction. Shortly downstream, there is a compound “gooseneck” meander bend called Polly’s Bend.

Google EarthTM image of Polly’s Bend. The maximum width from tip to tip is ~ 5 km.; minimum width of the neck is ~ 350 - 400 m. 

While not the norm, such tight bends are not uncommon in winding alluvial rivers, and will eventually be cut off during a flood, when the channel cuts across the narrow neck. Polly’s Bend, however, is entrenched in bedrock. The narrow neck (and the rest of the bend) has more than 100 m of solid limestone bedrock to cut through. So a classic meander cutoff, with flow going overbank across the neck and cutting a new channel; that ain’t gonna happen.

Landforms as Extended Composite Phenotypes

Submitted by jdp on Wed, 07/15/2015 - 01:27 pm

The online version of my new article exploring biogeomorphology from the perspective of niche construction and extended phenotypes is now out. The abstract is below. I appreciate my colleague Daehyun Kim encouraging me to stick with some of the more speculative and provocative ideas here. I was about to back off from them at one point, but he encouraged me to go for it.

Reference: Phillips, J.D. 2015. Landforms as extended compositive phenotypes. Earth Surface Processes and Landforms DOI: 10.1002/esp.3764.

 

 

Quo vadis, Physical Geography? Part 2

Submitted by jdp on Tue, 07/07/2015 - 12:18 pm

First part is here. 

An oversimplified, drive-by version of the changing role of physical geography includes these overlapping and not mutually exclusive stages:

1. Discovery and exploration—collecting basic data and observations on topography, geology, biota, meteorology, oceanography, etc., often in conjunction with surveying, mapping, and collection of anthropological and economic data. In this stage physical geographers are simply, but not exclusively, Geographers. They are also, in various cases, anthropologists, biologists, ethnographers, geologists, meteorologists, oceanographers, and surveyors.

2. Holding up the Earth and environmental sciences end of the integrated geographical analysis of places, regions, and various geographical systems (e.g., transportation and settlement patterns, trade networks, cultural landscapes, climate zones, biomes, agricultural systems, etc.). Physical geographers in this stage were either specialists in the physical side of the discipline, or broadly trained geographers with substantive physical expertise.

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