Low Flow Season Research Articles

Long term prediction of flood occurrence

Abstract. How long a river remembers its past is still an open question. Perturbations occurring in large catchments may impact the flow regime for several weeks and months, therefore providing a physical explanation for the occasional tendency of floods to occur in clusters. The research question explored in this paper may be stated as follows: can higher than usual river discharges in the low flow season be associated to a higher probability of floods in the subsequent high flow season? The physical explanation for such association may be related to the presence of higher soil moisture storage at the beginning of the high flow season, which may induce lower infiltration rates and therefore higher river runoff. Another possible explanation is persistence of climate, due to presence of long-term properties in atmospheric circulation. We focus on the Po River at Pontelagoscuro, whose catchment area amounts to 71 000 km2. We look at the stochastic connection between average river flows in the pre-flood season and the peak flows in the flood season by using a bivariate probability distribution. We found that the shape of the flood frequency distribution is significantly impacted by the river flow regime in the low flow season. The proposed technique, which can be classified as a data assimilation approach, may allow one to reduce the uncertainty associated to the estimation of the flood probability.

Sources and transport of contaminants of emerging concern: A two-year study of occurrence and spatiotemporal variation in a mixed land use watershed

The occurrence and spatiotemporal variation of 26 contaminants of emerging concern (CECs) were evaluated in 68 water samples in 2011–2012 in the Zumbro River watershed, Minnesota, U.S.A. Samples were collected across a range of seasonal/hydrological conditions from four stream sites that varied in associated land use and presence of an upstream wastewater treatment plant (WWTP). Selected CECs included human/veterinary pharmaceuticals, personal care products, pesticides, phytoestrogens, and commercial/industrial compounds. Detection frequencies and concentrations varied, with atrazine, metolachlor, acetaminophen, caffeine, DEET, and trimethoprim detected in more than 70% of samples, acetochlor, mecoprop, carbamazepine, and daidzein detected in 30%-50% of samples, and 4-nonylphenol, cotinine, sulfamethoxazole, erythromycin, tylosin, and carbaryl detected in 10%–30% of samples. The remaining target CECs were not detected in water samples. Three land use-associated trends were observed for the detected CECs. Carbamazepine, 4-nonylphenol, erythromycin, sulfamethoxazole, tylosin, and carbaryl profiles were WWTP-dominated, as demonstrated by more consistent loading and significantly greater concentrations downstream of the WWTP and during low-flow seasons. In contrast, acetaminophen, trimethoprim, DEET, caffeine, cotinine, and mecoprop patterns demonstrated both seasonally-variable non-WWTP-associated and continual WWTP-associated influences. Surface water studies of CECs often target areas near WWTPs. This study suggests that several CECs often characterized as effluent-associated have additional important sources such as septic systems or land-applied biosolids. Finally, agricultural herbicide (atrazine, acetochlor, and metolachlor) profiles were strongly influenced by agricultural land use and seasonal application-runoff, evident by significantly greater concentrations and loadings at upstream sites and in early summer when application and precipitation rates are greatest. Our results indicate that CEC monitoring studies should consider a range of land uses, seasonality, and transport pathways in relation to concentrations and loadings. This knowledge can augment CEC monitoring programs to result in more accurate source, occurrence, and ecological risk characterizations, more precisely targeted mitigation initiatives, and ultimately, enhanced environmental decision-making.

Nonstationarity of low flows and their timing in the eastern United States

Abstract. The analysis of the spatial and temporal patterns of low flows as well as their generation mechanisms over large geographic regions can provide valuable insights and understanding for climate change impacts, regional frequency analysis, risk assessment of extreme events, and decision-making regarding allowable withdrawals. The goal of this paper is to examine nonstationarity in low flow generation across the eastern US and explore the potential anthropogenic influences or climate drivers. We use nonparametric tests to identify abrupt and gradual changes in time series of low flows and their timing for 508 USGS streamflow gauging sites in the eastern US with more than 50 years of daily data, to systematically distinguish the effects of human intervention from those of climate variability. A time series decomposition algorithm was applied to 1-day, 7-day, 30-day, and 90-day annual low flow time series that combines the Box–Ljung test for detection of autocorrelation, the Pettitt test for abrupt step changes and the Mann–Kendall test for monotonic trends. Examination of the USGS notes for each site showed that many of the sites with step changes and around half of the sites with an increasing trend have been documented as having some kind of regulation. Sites with decreasing or no trend are less likely to have documented influences on flows. Overall, a general pattern of increasing low flows in the northeast and decreasing low flows in the southeast is evident over a common time period (1951–2005), even when discarding sites with significant autocorrelation, documented regulation or other human impacts. The north–south pattern of trends is consistent with changes in antecedent precipitation. The main exception is along the mid-Atlantic coastal aquifer system from eastern Virginia northwards, where low flows have decreased despite increasing precipitation, and suggests that declining groundwater levels due to pumping may have contributed to decreased low flows. For most sites, the majority of low flows occur in one season in the late summer to fall, as driven by the lower precipitation and higher evaporative demand in this season, but this is complicated in many regions because of the presence of a secondary low flow season in the winter for sites in the extreme northeast and in the spring for sites in Florida. Trends in low flow timing are generally undetectable, although abrupt step changes appear to be associated with regulation.

River bank erosion and channel evolution in sand-bed braided reach of River Chenab: role of floods during different flow regimes

Braided reaches of large rivers in alluvial plains show major morphological changes, particularly the external bank erosion, due to the flood events. This paper highlights the bank erosion and channel evolution induced by eleven different flood events in a 7-km long reach of the River Chenab, Pakistan. The impact of floods on river bank erosion and channel evolution is analyzed under low and high flow conditions. Flood-induced changes, for river’s external banks and channel evolution, were assessed by processing Landsat ETM+ images in ArcGIS tool, and their inter-relationship is evaluated through regression analysis. The results revealed that the major morphological changes were triggered by the flood events occurred during the high flow or Monsoon season (July–September), whereas the flood events of similar magnitude occurring during low flow season (October–March) did not induce such changes. Mostly, the erosion remained limited to the middle part of the reach, where the branch channel flows along the bank. The average annual bank erosion rates are much higher as compared with a global scale. Data analysis showed a strong correlation between the mean high flows and total bank erosion indicating that Monsoon seasonal flows and floods are responsible for bank erosion. The present study further identifies the river bank locations highly susceptible to erosion by developing the correlation between bank erosion and branch channel progression. Strong correlation for bank erosion could be established with the shift of branch channels position flowing along the banks in braided reaches of sand bed rivers. However, the presence of sand bars along the river banks resulted in reduced erosion that weakens this relationship. The findings of the present study can help develop better understanding about the bank erosion process and constitute a key element to inform and improve river bank management.

Attraction efficiency in natural-like fishways according to weir operation and bed change in Nakdong River, Korea

In this study, the attraction efficiency in the natural-like fishways which were installed at the Dalseong and the Gangjeong-Goryeong weirs located at the Nakdong River of Korea was assessed according to riverbed changes, with or without the attraction flow, and operation of movable weirs. A two-dimensional physical habitat model, especially River2D, was used as a tool of the evaluation for the fishway efficiency. And weighted usable area (WUA) was eventually considered as the indicator of the attraction efficiency. The Zacco platypus which is dominant species of the Nakdong River was selected as a target fish. From the simulation results based on various scenarios, it confirmed that while operating the fishway of the Dalseong weir, the attraction efficiency increased only when the sluice gate of the movable weir was closed. Additionally, the attraction channel in its current form was not significantly effective in attracting the fish. In respect to the Gangjeong-Goryeong weir, it was confirmed that because of the bed change after the flooding, the efficiency of attraction to the fishway had been reduced sharply. The attraction efficiency was improved because the reverse flow heading to the entrance of the fishway was eliminated when the movable weir opening, although the degree of improvement in the efficiency was insignificant. Eventually, in order to increase the attraction efficiency of the Gangjeong-Goryeong weir, the movable weir was operated, even during the low flow season. An alternative could be considered, namely re-locating the fishway from left to right bank, where stable overflow occurs because of the fixed weir.

In-stream Physical Heterogeneity, Rainfall Aided Flushing, and Discharge on Stream Water Quality.

Implications of instream physical heterogeneity, rainfall-aided flushing, and stream discharge on water quality control have been investigated in a headwater stream of a climatic region that has contrasting dry and wet seasons. Dry (low flow) season's physical heterogeneity showed a positive correlation with good water quality. However, in the wet season, physical heterogeneity showed minor or no significance on water quality variations. Furthermore, physical heterogeneity appeared to be more complementary with good water quality subsequent to rainfall events. In many cases stream discharge was a reason for poor water quality. For the dry season, graywater inputs to the stream could be held responsible. In the wet season, it was probably the result of catchment level disturbances (e.g., regulation of ephemeral freshwater paths). Overall, this study revealed the importance of catchment-based approaches on water quality improvement in tandem with in-stream approaches framed on a temporal scale.

Vulnerability of groundwater resources to interaction with river water in a boreal catchment

Abstract. A low-altitude aerial infrared (AIR) survey was conducted to identify hydraulic connections between aquifers and rivers and to map spatial surface temperature patterns along boreal rivers. In addition, the stable isotopic compositions (δ18O, δD), dissolved silica (DSi) concentrations and electrical conductivity of water in combination with AIR data were used as tracers to verify the observed groundwater discharge into the river system in a boreal catchment. Based on low temperature anomalies in the AIR survey, around 370 groundwater discharge sites were located along the main river channel and its tributaries (203 km altogether). On the basis of the AIR survey, the longitudinal temperature patterns of the studied rivers differed noticeably. The stable isotopes and DSi composition revealed major differences between the studied rivers. The groundwater discharge locations identified in the proximity of 12 municipal water intake plants during the low-flow seasons should be considered as potential risk areas for water intake plants during flood periods (groundwater quality deterioration due to bank infiltration), and should be taken under consideration in river basin management under changing climatic situations.

Instream cover and shade mediate avian predation on trout in semi‐natural streams

AbstractPiscivory by birds can be significant, particularly on fish in small streams and during seasonal low flow when available cover from predators can be limited. Yet, how varying amounts of cover may change the extent of predation mortality from avian predators on fish is not clear. We evaluated size‐selective survival of coastal cutthroat trout (Oncorhynchus clarkii clarkii) in replicated semi‐natural stream sections. These sections provided high (0.01 m2 of cover per m2 of stream) or low (0.002 m2 of cover per m2 of stream) levels of instream cover available to trout and were closed to emigration. Each fish was individually tagged, allowing us to track retention of individuals during the course of the 36‐day experiment, which we attributed to survival from predators, because fish had no other way to leave the streams. Although other avian predators may have been active in our system and not detected, the only predator observed was the belted kingfisher Megaceryle alcyon, which is known to prey heavily on fish. In both treatments, trout >20.4 cm were not preyed upon indicating an increased ability to prey upon on smaller individuals. Increased availability of cover improved survival of trout by 12% in high relative to low cover stream sections. Trout also survived better in stream sections with greater shade, a factor we could not control in our system. Collectively, these findings indicate that instream cover and shade from avian predators can play an important role in driving survival of fish in small streams or during periods of low flow.

Metal bioaccumulation levels in different organs of three edible fish species from the river Ravi, Pakistan

Metals bioaccumulation in five organs of Cirrhinus mrigala , Labeo rohita and Catla catla captured from three industrial and sewage polluted downstream sites (Shahdera = B, Sunder = C and Balloki = D) were compared with a non-industrial upstream site (Siphon = A) during high (post monsoon) and low (winter) flow seasons of river Ravi. Mean concentrations of metals were significantly higher in low flow than the high flow season. Pattern of metal accumulation in the studied organs was: Zn > Fe > Mn > Cu > Cr > Pb > Ni > Hg > Cd. Kidneys showed mostly greater metal bioaccumulation than intestines, hearts, eyes and gills. Among fish species, the highest concentrations (µg/g dry weight) of Cr (3.77), Zn (56.22), Mn (8.95), Ni (1.70) and Hg (1.60) and lowest of Pb (2.53) were detected in C. mrigala whereas Cu (7.19), Fe (62.11) and Pb (2.64) appeared higher while Zn (52.69), Mn (7.82) and Ni (1.41) with lowest concentrations in C. catla . In contrast, lower concentrations of Cd (0.15), Cr (3.16), Cu (7.06) and Fe (54.18) were recorded in L. rohita . Accumulation of the metals was significantly different in organs among the different sampling sites. Based on metals accumulation pattern, second downstream site (Sunder) identified as the most polluted site due to untreated industrial and municipal discharges. Measured elevated levels of metals concentrations in fish organs indicated potential health risks for the fish and the food chain.

Comparison of artificial neural networks and autoregressive model for inflows forecasting of Roseires Reservoir for better prediction of irrigation water supply in Sudan

ABSTRACTThe Blue Nile River is utilized in Sudan as the main source of irrigation water. However, the river has a long, dry, low-flow season (October–May), which necessitates the use of regulations and rules to manage its water use during this period. This depends on the use of accurate lead time forecasts of inflows to the reservoirs built along the river. Thus a reliable and tested forecasting tool is needed to provide inflow forecast, with sufficient lead time. In the present study, artificial neural network (ANN) is used to model the recession curve of the flow hydrograph at El-Deim gauging station, which subsequently is used as inflows to the Roseires Reservoir on the Blue Nile River. Different scenarios of ANN have been tested to forecast 23 10-day mean discharges during the recession period and their performances were assessed. Results from the optimal ANN model were compared to those simulated with an autoregressive (AR1) model to check their accuracy. Modelling results showed that the ANN model developed is capable of accurately forecasting the inflows to the Roseires Reservoir and outperforms the AR1 model. It has then proposed for use in operation of the reservoir for purposes of predicting irrigation water supply.

The role of the geophysical template and environmental regimes in controlling stream-living trout populations

The importance of multiple processes and instream factors to aquatic biota has been explored extensively, but questions remain about how local spatiotemporal variability of aquatic biota is tied to environmental regimes and the geophysical template of streams. We used an individual-based trout model to explore the relative role of the geophysical template versus environmental regimes on biomass of trout (Oncorhynchus clarkii clarkii). We parameterized the model with observed data from each of the four headwater streams (their local geophysical template and environmental regime) and then ran 12 simulations where we replaced environmental regimes (stream temperature, flow, turbidity) of a given stream with values from each neighboring stream while keeping the geophysical template fixed. We also performed single-parameter sensitivity analyses on the model results from each of the four streams. Although our modeled findings show that trout biomass is most responsive to changes in the geophysical template of streams, they also reveal that biomass is restricted by available habitat during seasonal low flow, which is a product of both the stream’s geophysical template and flow regime. Our modeled results suggest that differences in the geophysical template among streams render trout more or less sensitive to environmental change, emphasizing the importance of local fish–habitat relationships in streams.

Risk of impacts from extreme weather and climate in river-based tilapia cage culture in Northern Thailand

This paper analyses the direct impacts of extreme weather, high and low flow events, seasonality and other climate-related phenomena on river-based tilapia cage aquaculture in Northern Thailand. An interdisciplinary approach was taken, which included analysis of secondary water flow and meteorological data, quantitative and qualitative surveys of farmers' experiences with extreme events, and direct observations during critical periods. Findings show that extreme high and low flows adversely impact a substantial fraction of farms, causing damage to cages, fish deaths, slow growth and disease problems. Economic losses are significant and result in financial debt. Compensation and assistance following floods is modest relative to losses, and coverage is incomplete. Probability of extreme flows and impacts vary among locations, are highly seasonal, and vary inter-annually. These findings are important to improving management of climate-related risks, under both current and future climatic conditions.

Large-scale dam removal on the Elwha River, Washington, USA: Fluvial sediment load

The Elwha River restoration project, in Washington State, includes the largest dam-removal project in United States history to date. Starting September 2011, two nearly century-old dams that collectively contained 21±3millionm3 of sediment were removed over the course of three years with a top-down deconstruction strategy designed to meter the release of a portion of the dam-trapped sediment. Gauging with sediment-surrogate technologies during the first two years downstream from the project measured 8,200,000±3,400,000tonnes of transported sediment, with 1,100,000 and 7,100,000t moving in years 1 and 2, respectively, representing 3 and 20 times the Elwha River annual sediment load of 340,000±80,000t/y. During the study period, the discharge in the Elwha River was greater than normal (107% in year 1 and 108% in year 2); however, the magnitudes of the peak-flow events during the study period were relatively benign with the largest discharge of 292m3/s (73% of the 2-year annual peak-flow event) early in the project when both extant reservoirs still retained sediment. Despite the muted peak flows, sediment transport was large, with measured suspended-sediment concentrations during the study period ranging from 44 to 16,300mg/L and gauged bedload transport as large as 24,700t/d. Five distinct sediment-release periods were identified when sediment loads were notably increased (when lateral erosion in the former reservoirs was active) or reduced (when reservoir retention or seasonal low flows and cessation of lateral erosion reduced sediment transport). Total suspended-sediment load was 930,000t in year 1 and 5,400,000t in year 2. Of the total 6,300,000±3,200,000t of suspended-sediment load, 3,400,000t consisted of silt and clay and 2,900,000t was sand. Gauged bedload on the lower Elwha River in year 2 of the project was 450,000±360,000t. Bedload was not quantified in year 1, but qualitative observations using bedload-surrogate instruments indicated detectable bedload starting just after full removal of the downstream dam. Using comparative studies from other sediment-laden rivers, the total ungauged fraction of <2-mm bedload was estimated to be on the order of 1.5Mt.

The O and H isotope characteristics of water from major rivers in China

Isotopic compositions of the water sources and evapotranspiration in large rivers of China are not well explored. In this investigation, δ18O and δD values of water from major large rivers of China are integrated to understand the spatial and temporal variations in the isotopes of water and d-excess. River water samples were collected from the channels and tributaries of the Changjiang, Yellow, Songhua, and Liao Rivers in China. Water from upstream was generally enriched with light isotopes relative to that from downstream areas. Water from the Songhua River and upper reach of the Changjiang River has relatively negative isotopic values, impacted by latitude and altitude effects, respectively. The spatial pattern showed that δ18O-H2O values in rivers mainly ranged from −9 to −7 ‰ in most of China based on roughly mapped contours, and rivers enriched in light isotopes were primarily located in the Tibetan Plateau and northeast China. The slopes of linear regression from the Yellow and Liao Rivers suggested that the loss of water by direct evaporation shifted the isotopes of water, especially in some small tributaries. Additionally, there was a narrow range of low δ18O-H2O values from the main channel of rivers during the low-flow season relative to the high-flow season due to influence of the monsoon climate. No distinct spatial variation of d-excess values was found in the rivers of China. However, water samples in some rivers with low d-excess values had high δ18O values, indicating that evaporation in the tributaries has led to significant water loss, especially in tributaries of the Yellow and Liao Rivers.

Accounting for environmental flow requirements in global water assessments

Abstract. As the water requirement for food production and other human needs grows, quantification of environmental flow requirements (EFRs) is necessary to assess the amount of water needed to sustain freshwater ecosystems. EFRs are the result of the quantification of water necessary to sustain the riverine ecosystem, which is calculated from the mean of an environmental flow (EF) method. In this study, five EF methods for calculating EFRs were compared with 11 case studies of locally assessed EFRs. We used three existing methods (Smakhtin, Tennant, and Tessmann) and two newly developed methods (the variable monthly flow method (VMF) and the Q90_Q50 method). All methods were compared globally and validated at local scales while mimicking the natural flow regime. The VMF and the Tessmann methods use algorithms to classify the flow regime into high, intermediate, and low-flow months and they take into account intra-annual variability by allocating EFRs with a percentage of mean monthly flow (MMF). The Q90_Q50 method allocates annual flow quantiles (Q90 and Q50) depending on the flow season. The results showed that, on average, 37% of annual discharge was required to sustain environmental flow requirement. More water is needed for environmental flows during low-flow periods (46–71% of average low-flows) compared to high-flow periods (17–45% of average high-flows). Environmental flow requirements estimates from the Tennant, Q90_Q50, and Smakhtin methods were higher than the locally calculated EFRs for river systems with relatively stable flows and were lower than the locally calculated EFRs for rivers with variable flows. The VMF and Tessmann methods showed the highest correlation with the locally calculated EFRs (R2=0.91). The main difference between the Tessmann and VMF methods is that the Tessmann method allocates all water to EFRs in low-flow periods while the VMF method allocates 60% of the flow in low-flow periods. Thus, other water sectors such as irrigation can withdraw up to 40% of the flow during the low-flow season and freshwater ecosystems can still be kept in reasonable ecological condition. The global applicability of the five methods was tested using the global vegetation and the Lund-Potsdam-Jena managed land (LPJmL) hydrological model. The calculated global annual EFRs for fair ecological conditions represent between 25 and 46% of mean annual flow (MAF). Variable flow regimes, such as the Nile, have lower EFRs (ranging from 12 to 48% of MAF) than stable tropical regimes such as the Amazon (which has EFRs ranging from 30 to 67% of MAF).

Examining muscles of Cirrhinus mrigala for biochemical parameters as a bio-indicator of water pollution by municipal and industrial effluents into River Ravi, Pakistan

We examined the muscles from replicated samples of Cirrhinus mrigala as a bio-indicator of water pollution following sewage and industrial discharges from the Lahore city into Ravi River of Pakistan. For this, C. mrigala was netted from three variably polluted downstream sites (B, C and D) and one less polluted upstream site (A). The fish was sampled during low (winter) and high (post monsoon) river flow seasons. The fish muscles from site D showed 28 % less total carbohydrates than the fish caught from site A. The muscle carbohydrate contents were further decreased at sites B and C by 58 and 59 % and 77 and 74 %, during low and high flows, respectively. Moreover, total lipids, cholesterol and RNA contents for the muscle of the fish from the downstream locations were also decreased up to 29, 68 and 58 %, respectively. Con- versely, total protein, soluble protein and DNA contents were increased in fish muscles progressively downstream during both the low- and high-flow seasons. It appeared that the fish muscle components can change with the level of water pollution which may affect the fish attributes as a safe food.

Assessment of chlorophyll-a variations in high- and low-flow seasons in Apalachicola Bay by MODIS 250-m remote sensing.

Chlorophyll-a (chl-a) is considered as a primary indicator for water quality and foods for oyster growth in Apalachicola estuarine ecosystem. Assessment of chl-a concentration variation in response to river inflow is important for estuarine environmental research and management. In this study, remote sensing analysis has been conducted to evaluate the effects of river inflow on chlorophyll concentrations in Apalachicola Bay of Florida in the northeast Gulf of Mexico. A remote sensing model for chl-a was improved and applied to map spatial distributions of chl-a by using Moderate Resolution Imaging Spectroradiometer (MODIS) 250-m resolution imageries in high-flow and low-flow seasons in 2001 and 2008. Chl-a values approximately ranged from the minimum 6μg/l to the maximum 29μg/l in the study period. Maximum chl-a concentration in high-flow season was almost twice above that in low-flow season. The averaged mean and minimum chl-a level in the high-flow season were approximately 42 and 28% higher than those in low-flow season, respectively. The remote sensing mapping of chl-a was able to show spatial variations of chl-a in the entire bay under different flow conditions, which indicated its advantage over the traditional field data sampling for monitoring water quality over a large area of estuary. The MODIS 250-m remote sensing regression model presented from this study can be used to support monitoring and assessment of the spatial chl-a distribution in the bay for environmental research and management in Apalachicola Bay.

부영양화된 하천형 호소(팔당호) 전이대의 수질모델링

This study was conducted to investigate the main cause of water quality deterioration during the spring season in the transition zone between the South Han River and the river-reservoir Paldang. A water quality model modified from QUAL2E (U.S.EPA) was used, and the model showed that eutrophication and algal production in the low flow season affected about 60% of the organic pollution at the downstream of the South Han River. This result means that phosphorus control is prior to external organic material management to ameliorate the deterioration of water quality in the water body.

Analysis of δ15N and δ18O to identify nitrate sources and transformations in Songhua River, Northeast China

To understand the sources and transformations of nitrate in the Songhua River basin, which is one of seven largest river basins in China, the concentration of dissolved nitrogenous species, nitrogen and oxygen isotopes of NO3−, nitrogen isotopes of NH4+, and stable isotopes of water were determined in this study. Low NO3− concentrations and a high dissolved organic nitrogen/total dissolved nitrogen ratio (DON/TDN) were observed in the Nen River and other rivers originating from the mountains, which are covered by forest. NO3− and DON were the major nitrogenous compounds in aquatic systems, accounting for the TDN being about 90% during high flow season and about 85% during low flow season, respectively. The nitrogen efflux for the entire basin was estimated to be approximately 1.17 × 105 tons/yr, which represents an annual N output of 0.21 ton/(km2 yr). The majority of the δ18O-NO3− values were between −4‰ and 4‰, reflecting nitrification. During the high flow season, the isotopic compositions of NO3− and the water chemistry suggest that NO3− in the Nen River was mainly derived from soil organic nitrogen (SON), whereas NO3− in the Songhua River originated from organic nitrogen, nitrogenous fertilizers and sewage waters. NO3− in the low flow season samples generally originates from SON and sewage waters. Moreover, the calculated loss of nitrate via the mass budget in rivers, together with isotopic values and water chemistry confirm that denitrification occurs during the high flow season, especially in the Songhua River. This study suggests that the mass calculation and isotopic proof provide a better understanding for riverine N budget and biogeochemical processes.

Winter Flows in the Mackenzie Drainage System

Winter low flow of northern rivers refers to the diminished discharge between the time of rapid flow reduction in the freeze-up period and the arrival of spring freshet, when the flow makes a quick rise. For the Mackenzie River in Canada, the duration of the winter low-flow season so defined varies considerably within the river’s large basin (1.8 million km2); therefore, to give a common time frame that enables between-basin comparison we consider 1 November to 31 March as the winter flow season. Several hydroclimatic conditions influence winter flows to varying degrees. Lengthy periods of sub-freezing temper­atures inhibit rain events and prevent snowmelt, while the formation of river ice increases channel storage at the expense of discharge. Groundwater sustains baseflow, and the flow amount at most stations is related to autumnal discharge, which reflects groundwater storage status in the pre-winter season. Large reservoirs and lakes provide substantially higher winter flows than their neighboring non-lake areas. Winter flow increases downstream as more water is gathered from the expanded drainage network, but flow contribution varies: larger baseflow is delivered from uplands than from lowlands, and discharge from the Williston Lake reservoir, regulated for hydropower production, provides about half of the total winter flow of the Mackenzie. Monotonic linear trends in winter flow are detected statistically for some tributaries, but the effect of short-term flow variability and the confounding influence of managed flow should be evaluated when considering long-term tendencies and their causative factors.