Base Flow Regime Research Articles

Pesticides, Polycyclic Aromatic Hydrocarbons, and Polychlorinated Biphenyls in Transport in Two Atlantic Coastal Plain Tributaries and Loadings to Chesapeake Bay

Concentrations of current-use pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine (OC) insecticides were determined above the reach of tide in the Chesterville Branch and Nanticoke River on the eastern shore of Chesapeake Bay during base-flow and storm-flow hydrologic regimes to evaluate mass transport to Chesapeake Bay. The two rivers monitored showed relatively high concentrations of atrazine, simazine, alachlor, and metolachlor in comparison to previously investigated western shore tributaries, and reflected the predominant agricultural land use in the eastern shore watersheds. The four current use pesticides showed the greatest seasonal contribution to annual loadings to tidal waters of Chesapeake Bay from the two rivers, and the relative order of annual loadings for the other contaminant classes was PAHs>PCBs>OC insecticides. Annual loadings normalized to the landscape areas of selected Chesapeake Bay watersheds showed correlations to identifiable source areas, with the highest pesticide yields (g/km2/yr) occurring in eastern shore agricultural landscapes, and the highest PAH yields derived from urban regions.

Groundwater-related thallium transfer processes and their impacts on the ecosystem: southwest Guizhou Province, China

The small karstic watershed of Lanmuchang, in a Hg–Tl mineralized area in SW Guizhou Province, China, exhibits an enrichment of toxic Tl in groundwater and related stream water. This affords an excellent demonstration of the natural processes of Tl dispersion, and the resultant impact on the local ecosystem. The distribution of Tl in the water system follows a decreasing concentration pattern from deep groundwater to stream water to shallow groundwater. Tl shows high levels (13–1100 μg/L) in deep groundwater within the Tl-mineralized area, decreasing with distance away from the mineralized area to background levels (0.005 μg/L). The distribution of Tl in the water system is constrained by Tl mineralization, water–rock interactions and hydrogeological conditions. Tl concentrations in waters generally correlate with concentrations of total dissolved solids, sulphate, Ca and pH values, suggesting the contribution of water-rock interactions to water geochemistry. Water–rock interactions are driven by weathering of Tl-bearing sulfides which decreases pH values in groundwater, and by dissolution of limestone enhanced by acid fluids. Tl in stream water in both the base-flow and flood-flow regimes shows higher concentrations than it does in shallow groundwater that serves as the stream's source (mainly springs, dug-well flows and karstic cave waters). Concentrations of Tl in stream water in the flood-flow regime are generally lower than in the base-flow regime due to dilution effects, but those in the waters of mid-stream are almost the same as in the base-flow regime, probably due to contribution from Tl-rich soil water seepage or from acid mine drainage (AMD). Unexpectedly, Tl concentrations in stream water in both regimes are remarkably higher (2–30 fold) downstream than up- and mid-stream. These pronounced increases of Tl concentration are likely caused by unidentified discharges of deep groundwater through fractured zones to the downstream trace. The groundwater-related Tl transfer processes affect the ecosystem through contamination of water supply and arable soil and ultimately the food chain with undoubted risks to human health. Therefore, the results of this study are important for environmental planning and regulations, and will also serve as baseline data for future research on Tl natural dispersion processes.

Flow management strategies to control blooms of the cyanobacterium, Anabaena circinalis, in the River Murray at Morgan, South Australia

AbstractThe Murray–Darling river system is highly regulated and is Australia's major surface water resource. It is subject to blooms of the toxic cyanobacterium Anabaena circinalis, which present significant water quality problems. As a result of these blooms, an algal management strategy has been developed for the Murray–Darling basin. One of the major objectives of the strategy is the development of flow management strategies for key reaches of the river system. Intensive studies in the Murrumbidgee River, Australia, have indicated that persistent thermal stratification is a requirement for blooms of this cyanobacterium to occur. In the lower Murray, mean wind speed was found to be the major factor affecting the degree of thermal stratification under low flow conditions, which generally exist during the months of December to March. In this paper, the effect of various flow management scenarios on the likelihood of the occurrence of blooms in the River Murray at Morgan, South Australia, are assessed. A frequency analysis is carried out on 30 years of wind speed data to determine the probability of occurrence of persistent thermal stratification under a number of flow regimes. The scenarios evaluated include existing base flow conditions, altered base flow regimes, temporary releases from an upstream storage (Lake Victoria) and the temporary reduction of weir pool levels. The results obtained indicate that the dispersal of existing blooms by simultaneously reducing the weir pool levels at Locks 1–3 is the most effective and economical strategy for combating bloom formation by A. circinalis in the River Murray at Morgan. Copyright © 2001 John Wiley & Sons, Ltd.

Influence of stream flow regime and temperature on growth rate of the riparian tree, Platanus wrightii, in Arizona

1. The growth of riparian trees in semi‐arid regions is influenced by stream flow regime, but the relative importance of base flow and seasonal floods on growth has not been explored. I examined abiotic influences on the growth of Platanus wrightii in four stream reaches in Arizona. All reaches had a bimodal pattern of discharge, but only two had continuous flow throughout the growing season. 2. In two reaches of Sycamore Creek without perennial flow, a large percentage of the annual variation in radial growth rate of P. wrightii was explained by annual and growing season flow rate. Growth was related to these same variables in a perennial reach of Sycamore Creek, but trees maintained higher growth during drought years than they did in the temporary reaches. At Oak Creek, a larger perennial stream, P. wrightii growth showed a bell‐shaped relationship with flow. These data suggest that growth rate is frequently limited by water availability at Sycamore Creek, but not at Oak Creek. 3. At both rivers, much of the annual surface flow occurs as winter floods. Oak Creek, however, maintains a high summer base flow even during years with no floods. Platanus wrightii growth was significantly related to winter flood frequency only at Sycamore Creek. The positive relationship of growth with stream flow and winter flood frequency at Sycamore Creek presumably occurs because the P. wrightii trees are dependent on the winter flows to recharge the shallow alluvial aquifer and to raise the level of ground water within the root zone. 4. Frequent summer floods increased the growth of trees in perennial and non‐perennial reaches alike. At perennial Oak Creek, summer flood frequency was the only variable linearly related to growth of P. wrightii. Summer flood frequency was a significant, but secondary, component of multiple‐regression growth models for trees in the perennial and non‐perennial reaches of Sycamore Creek. Summer floods may stimulate growth, in part, by replenishing limiting nutrients. 5. High temperature was negatively associated with the growth of P. wrightii at Sycamore Creek. The combination of drought and high temperature resulted in very low growth rate. 6. These results have implications for the management of flood and base flow regimes on regulated, diverted and pumped rivers.

HYDROLOGIC EFFECTS OF THE TONKAWA CREEK FLOOD ABATEMENT PROGRAM1

ABSTRACT: Floodwater‐retarding impoundments, controlling 68 percent of the drainage area of Tonkawa Creek, a Washita River tributary in southwestern Oklahoma, have reduced the total flow volume about 36 percent over a 5‐year period. Analyses showed the reduction occurred primarily in the less‐than‐2.5‐cfs flow range, indicating the base flow regime has been altered. However, channelizing the downstream, mild‐sloped, 3.6 miles of Tonkawa Creek that flows across a Washita River terrace increased the flow volume fourfold at the outlet. A double‐masscurve analysis of water yield from a 1,127‐square‐mile Washita basin segment versus an untreated tributary showed the yield has not changed after 25 percent of the tributary area had been treated. Therefore, the flow reduction caused by structures is being offset by increased yields from channelization.

Hydrology of 7-Day 10-Yr Low Flows

The level of treatment necessary for maintaining stream quality depends on the dilution ratio, defined as the ratio of the 7-day 10-yr low flow in the stream to the average dry weather flow from the treatment plant. The natural low flows depend on physiography, baseflow regime, stream entrenchment, flood-plain development, magnitude of the drainage area of the stream, etc. The natural low flows are modified by recreational pools created by in-river impoundments, reservoirs or lakes for recreation and regulation of flows, urbanization, municipal and industrial water use and wastewater treatment plant effluents, and navigation pools. Downstream of the confluence of two major streams draining hydrologically dissimilar areas, the low flow exceeds the sum of the two individual low flows, because they are out of phase with each other. Examples are presented showing the effect of these factors on the 7-day 10-yr low flow in streams.