The geographic distribution of small dams in Oregon using ecoregion and landform classification

Abstract

Small dams fragment river landscapes, disrupting channel connectivity and impacting water quality and quantity. Although they impound volumetrically less total water than large dams, the ubiquity of small dams suggests their cumulative impacts could be significant. Documenting the distribution and characteristics of small dams is necessary to understanding and mitigating their impacts. In this study, we compare datasets of small dams in Oregon, compile a new comprehensive dataset, and document geographic variations in small dam distributions between different ecoregions. We used Oregon Water Resources Department dam and Oregon Department of Fish and Wildlife fish passage barrier datasets to compare dam heights and contributing drainage areas between different ecoregions. Over 61% of Oregon’s land area is above one or more small dam. We highlight the location of Oregon’s small dams at valley margins, transition zones between flat plains and mountains, and areas of high population density. Given the hidden nature of small dams, evaluation of small dam impacts using public imagery is not effective. However, knowledge of small dam distributions given their association with landforms can aid in finding unrecorded dams, assessing approaches for evaluating their geomorphic impacts, and informing their management.     

Methods for monitoring tidal flushing in large animal burrows in tropical mangrove swamps

The typically anaerobic nature of mangrove sediments provides significant challenges to the mangrove trees and biota inhabiting them. The burrowing activities and flow of water through the numerous and complex animal burrows perforating the sediments of mangroves have a major influence on the biogeochemistry of the sediments and are important to the enhancement of nutrient and oxygen exchange. Two new methods are presented for monitoring the tidal flushing of Sesarma messa and Alpheus cf macklay burrows in a Rhizophora stylosa mangrove forest – by measuring oxygen content of burrow water and by determining the change in fluorescence of a dye tracer through tidal inundation. A case study using the first of these showed oxygen consumption rates at the burrow wall deep within the burrow were found to be between 210 and 460 μmol O₂ m⁻² h⁻¹. The influx of oxygen during a flood tide was found to be significant and indicated that approximately 40% of the burrow water is flushed during a single tidal event. However, the high consumption rate of oxygen within the burrow resulted in the oxygen concentration remaining at or below one-third of the oxygen content of the flooding tidal water. A test application of the second method, using rhodamine dye as a tracer, indicated that the exchange of water between the burrow and the flooding tide was found to be in the order of 30% of the burrow volume. These new techniques provide a means to further study the nutrient exchange within these burrow systems and verify the initial findings that several tidal inundations are necessary to completely flush the burrows.     

Annual estimates of water and solute export from 42 tributaries to the Yukon River

Annual export of 11 major and trace solutes for the Yukon River is found to be accurately determined based on summing 42 tributary contributions. These findings provide the first published estimates of tributary specific distribution of solutes within the Yukon River basin. First, we show that annual discharge of the Yukon River can be computed by summing calculated annual discharges from 42 tributaries. Annual discharge for the tributaries is calculated from the basin area and average annual precipitation over that area using a previously published regional regression equation. Based on tributary inputs, we estimate an average annual discharge for the Yukon River of 210 km³ year^–1. This value is within 1% of the average measured annual discharge at the U.S. Geological Survey gaging station near the river terminus at Pilot Station, AK, for water years 2001 through 2005. Next, annual loads for 11 solutes are determined by combining annual discharge with point measurements of solute concentrations in tributary river water. Based on the sum of solutes in tributary water, we find that the Yukon River discharges approximately 33 million metric tons of dissolved solids each year at Pilot Station. Discharged solutes are dominated by cations calcium and magnesium (5.65 × 10⁹ and 1.42 × 10⁹ g year^–1) and anions bicarbonate and sulphate (17.3 × 10⁹ and 5.40 × 10⁹ g year^–1). These loads compare well with loads calculated independently at the three continuous gaging stations along the Yukon River. These findings show how annual solute yields vary throughout a major subarctic river basin and that accurate estimates of total river export can be determined from calculated tributary contributions. Copyright © 2011. This article is a U.S. Government work and is in the public domain in the USA.     

Quantifying preferential flow in soils: A review of different techniques

Preferential flow (PF) in soil has both environmental and human health implications since it favours contaminant transport to groundwater without interaction with the chemically and biologically reactive upper layer of soil. PF is, however, difficult to measure and quantify. This paper reviews laboratory and field techniques, such as breakthrough curves, dye tracing, and scanning techniques, for evaluating PF in soil at different scales. Advanced technologies, such as scanning techniques, have increased our capability to quantify transport processes within the soil with minimal soil disturbance. Important issues with respect to quantifying PF concern large-scale studies, frozen soil conditions, tracing techniques for particles and gases, a lack of simple mathematical tools for interpreting field data, and the lack of a systematic approach for comparing PF data resulting from different measurement techniques. Also, more research is required to quantify the relative importance of the various PF processes that occur in soil rather than the integrated result of all PF processes in soils.     

Emplacement ages and sources of kimberlites and related rocks in southern Africa: U–Pb ages and Sr–Nd isotopes of groundmass perovskite

Groundmass perovskite has been dated by LA-ICPMS in 135 kimberlites and related rocks from 110 localities across southern Africa. Sr and/or Nd isotopes have been analysed by LA-MC-ICPMS in a subset of these and integrated with published data. The age distribution shows peaks at 1,600–1,800, 1,000–1,200, 500–800 and 50–130 Ma. The major “bloom” of Group I kimberlites at ca 90 ± 10 Ma was preceded by a slow build-up in magmatic activity from ca 180 Ma. The main pulse of Group II kimberlites at 120–130 Ma was a distinct episode within this build-up. Comparison of the isotopic data with seismic tomography images suggests that metasomatized subcontinental lithospheric mantle (SCLM) with very low ε _Nd and high ⁸⁷Sr/⁸⁶Sr, (the isotopic signature of Group II kimberlites) was focused in low-Vs zones along translithospheric structures. Such metasomatized zones existed as early as 1,800 Ma, but were only sporadically tapped until the magmatic build-up began at ca 180 Ma, and contributed little to the kimberlitic magmas after ca 110 Ma. We suggest that these metasomatized volumes resided in the deep SCLM and that their low-melting point components were “burned off” by rising temperatures, presumably during an asthenospheric upwelling that led to SCLM thinning and a rise in the ambient geotherm between 120 and 90 Ma. The younger Group I kimberlites therefore rarely interacted with such SCLM, but had improved access to shallower volumes of differently metasomatized, ancient SCLM with low ⁸⁷Sr/⁸⁶Sr and intermediate ε _Nd (0–5). The kimberlite compositions therefore reflect the evolution of the SCLM of southern Africa, with metasomatic-enrichment events from as early as 1.8 Ga, through a major thermal and compositional change at ca 110 Ma, and the major kimberlite “bloom” around 90 Ma.