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The breakup of the ice cover in cold-region rivers is a brief but seminal period of their hydrologic regime, with important ecological and socio-economic implications. The main driver of ice breakup processes is the flow discharge hydrograph. It is generated by runoff from snowmelt and rainfall but can be modified by rapid release of water from storage as the ice cover recedes by ablation and mechanical breaking up. Despite its potential importance, there is very limited quantitative information concerning the hydrodynamic processes that control storage release during ice breakup in rivers, while the issue of climate change underscores the need for improved understanding of the relevant mechanisms. Quantitative analysis for assumed prismatic channels shows that ablation and sustained ice dislodgment and breaking can cause significant flow enhancement via storage release. The latter process is far more dynamic than ablation and, under certain conditions, may lead to formation of a self-sustaining wave (SSW). Analytical results are applied to natural stream conditions, using the Lower Peace River as a case study. Observed rates of ice recession typically indicate ice melt as the dominant process. A rare occurrence of rapid ice breaking over hundreds of kilometres (2014) indicated partial agreement with the SSW concept and revealed that discrepancies may arise from the characteristic irregularity of rivers. Impacts on storage release by climate-driven changes to river ice regimes are examined, along with their implications to ice-jam formation and associated flooding. [ABSTRACT FROM AUTHOR]

River concentrations of suspended sediment and particulate contaminants, such as trace metals, increase sharply during ice breakup, with potentially detrimental ecological impacts that may be complicated by changing climatic conditions. To enhance the very limited knowledge on this issue, comprehensive data have been collected on the Saint John River (SJR). During breakup, the suspended sediment concentration (SSC) was found to rise gradually, crest, and decline, roughly in step with the runoff, but occasionally spiked to extremely high peaks. The latter ranged from 4.2 to 6.5 times the runoff-generated peak concentrations (RPCs), which ranged from 35 to 150 mg/L. Peak RPCs and individual-event sediment loads generally increased with flow discharge. The sediment spikes were invariably associated with waves resulting from releases of upstream ice jams and with the ensuing ice runs. Concentration–discharge graphs exhibited pronounced clockwise hysteresis, indicative of sediment supply constraints. This feature is more prominent in high-runoff events and typically associated with a lag of 1–3 days between peak concentration and peak discharge, which arrives later. Prediction of SSC via sediment-rating curves is hopeless, but such curves can be helpful in computing loads associated with individual events using a modified approach that terminates load computation 3 days after the arrival of peak discharge. The bulk of the sediment load is delivered on the rising limb of the hydrograph and is likely to be missed in routine sediment monitoring programs. Practical steps to capture this information are suggested. Increases in SJR spring flows during recent decades are projected to continue under a warming climate, resulting in considerable increases of SSCs and loads by the end of this century. [ABSTRACT FROM AUTHOR]

Highly dynamic flow conditions that often occur during the river ice breakup can cause greatly increased in-stream sediment concentrations and loads, with significant impacts on aquatic ecology and the environment. Sharp waves accompanying the releases of ice jams cause transient but very high flow velocities and shear stresses. However, very limited quantitative information is available on how such ice jam-generated waves (or “javes” for short) influence the transport of suspended sediment even though their effects on it are visually apparent. Suspended sediment concentrations (SSCs), obtained during a 5-year study of ice and sediment processes in the Saint John River (NB), reveal occasional spikes in concentration, consisting of relatively brief upward excursions to nearly order-of-magnitude higher values. Such spikes are concurrent with javes and ice runs. Documented cases of sediment pulses during river ice breakup events in the Saint John River form a unique data set demonstrating temporal and spatial characteristics. Peak SSC values approach 1 g/L and approximately coincide with peak jave stages and surface ice concentrations. Pulse durations amount to several hours, and are shorter than those of the concomitant javes. Despite their brevity, pulses can deliver sediment loads that are significant fractions of the total load for the entire breakup event. Measurement of breakup sediment pulses is not feasible under existing monitoring programs. Instead, focused short-term studies could lead to development of predictive capability, linking pulse properties to hydro-climatic variables that drive the breakup event. A strong influence of river discharge has been detected, leading to projected increases in pulse sediment amounts under a changing climate. [ABSTRACT FROM AUTHOR]

The Peace-Athabasca Delta (PAD) in northern Alberta is one of the world's largest inland freshwater deltas, home to large populations of waterfowl, muskrat, beaver, and free-ranging wood bison. Following construction of the Bennett Dam in the late 1960s, a paucity of ice-jam flooding in the lower Peace River has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life in the PAD region. Past studies have shown that both regulation and climate have contributed to the drying trend, via increased freezeup levels and reduced breakup flows, respectively. However, it has not so far been possible to assess the relative impacts of these two factors. This question is addressed herein by developing quantitative relationships based on the concept of conditional probability. The new methodology is first tested against observed frequencies of ice-jam floods both before and after regulation; it is subsequently applied to quantify the effects of climate and of regulation on the post-regulation reduction in ice-jam flood frequency. The results indicate that both factors have contributed significantly to the drying of the PAD, with regulation having had the more pronounced effect. Complications that may arise from secondary effects of regulation on breakup flows or of climate on freezeup levels are discussed and quantified. It is shown that such effects do not alter the overall conclusions of the paper. [ABSTRACT FROM AUTHOR]

Abstract: Ice-jam release waves, or javes for short, are highly dynamic events that occur in ice-forming rivers. They are known for serious ecological and socio-economic impacts, which are usually detrimental but on occasion beneficial, particularly with respect to ecosystem maintenance in the large freshwater deltas of northern Canada. Detailed water level data have been obtained during the passage of javes in the past 10years or so, but their study is hampered by lack of velocity and discharge measurements, owing to the presence of moving ice in the flow. A previously developed method to analytically compute such parameters, which is based on measured waveforms, has given encouraging results in several applications. Herein, this method is first subjected to comprehensive validation using numerical model output for a hypothetical jave in a prismatic channel. The analytical method is shown to perform adequately with respect to all hydrodynamic jave characteristics, such as celerity, velocity, discharge, and bed shear stress. Jave data from various rivers are then compiled in tabular form and it is shown via dimensional analysis that bulk predictions can be made in terms of jave height and rate of rise of the water level. Implications of the present findings to riverbed scour and to ecologically vital ice-jam flooding of flat rivers and deltas are explored and research challenges identified. [Copyright &y& Elsevier]

Abstract: Ice breakup is a controlling factor in the hydrology of arctic deltas, including the Mackenzie River Delta, which is characterized by a flat front and numerous channels and lakes. Ice-jam flooding replenishes delta lakes with essential water, sediment and nutrients. The present study, carried out under the auspices of the International Polar Year, aims to gather essential quantitative information for improving understanding of delta ice processes and associated prediction capability. Comprehensive observations and measurements, which were carried out during the 2007 and 2008 breakups, are described along with relevant analysis and data interpretation. The spacing of pre-breakup transverse cracks in delta channels is similar to that of rivers, typically ranging from 2 to 5 ice cover widths. Ice jamming in the delta is driven by major ice runs originating in the Mackenzie River and stalling in the upper reaches of Middle Channel. The data collected during the mechanical breakup event of 2008 permitted crude, but first-ever, quantification of flow re-distribution resulting from the formation of a Middle Channel jam. This jam also generated jams in East Channel and smaller distributaries. Examination of threshold velocity for ice block submergence indicated that, with notable exceptions, the expected kind of jam in delta channels is a surface accumulation of ice blocks and plates. This prediction is in accord with observations and hydrometric data at the Inuvik and Aklavik gauges. The use of portable loggers to record water level variations at selected sites proved to be a robust and effective technique for obtaining key hydraulic data. [Copyright &y& Elsevier]

Abstract: River ice jams can cause extreme flood events with major socio-economic and ecological impacts. A major practical question is how to quantify and assess ice-jam flood risk. Ideally, this question can be answered by means of historical ice-influenced water level peaks. More commonly, however, the available historical information is scarce and determination of ice-jam flood frequencies or probabilities must rely on a synthetic method. After noting that empirical evidence does not support the assumption of discrete stage outcomes, which is central to the existing methodology, a new synthetic method is developed. It hinges on the fact that the peak stage can take on any value between discharge-dependent envelopes and is thence called the distributed-function method, or DFM, and successfully tested by means of four case studies. It is shown further that the DFM produces more realistic results relative to those of the existing approach. Practical limitations are common to both methods and arise primarily from the need to use the peak runoff discharge, rather than the unknown value that prevails at the time of the peak stage. Resulting errors in probability estimates are evaluated in an extreme case and shown to be both conservative and tolerable. Analysis of additional case studies is recommended as a means of enhancing the utility of the DFM. [Copyright &y& Elsevier]

Abstract: Dynamic processes that occur during the breakup of river ice covers have important socio-economic and ecological impacts, but development of predictive capability is hampered by the brevity of the event and the ever-changing flow and ice conditions. The spatial and temporal variation of river water levels reflects the evolution of breakup processes and may be used to quantify their characteristics. However, various practical difficulties in measuring such variations have contributed to a scarcity of relevant data. A recently developed technique for remote recording of river levels involves pre-breakup deployment of portable pressure loggers, which are retrieved weeks or months later. Logger memory can accommodate high sampling frequencies, sufficient to capture very rapid water level changes. Application of this technique to document a highly dynamic breakup in the Saint John River, New Brunswick, resulted in an extensive data set. The processed logger output is described in detail and shown to furnish important insights on the chronology of breakup events. Analysis of the water level records that were obtained at different locations resulted in quantification of the hydrodynamic properties of waves generated by ice jam releases, also known as “javes”. Jave-induced amplification of flow and shear stress decreased with traveled distance, being between 2.5 and 3.0 at a distance of 11km. Extensive mobilization of the riverbed during the passage of javes in the study reach is likely. Application of a numerical model to the measured WL profile of a major ice jam resulted in default-range calibration coefficients, further corroborating current modeling capability. [Copyright &y& Elsevier]

Abstract: During the 1994 and 1995 spring breakup events, ice jams were formed on the lower Matapedia River and remained in place long enough to permit collection of detailed field data on jam thickness as well as on water surface elevations. Together with bathymetric data that were obtained in the following summers, the spring measurements form one of the most complete ice-jam data sets that have been obtained to date. This allowed examination of how well current understanding of the hydromechanics of ice jams, incorporated in a numerical model, can describe the results of the measurements. The availability of jam thickness data was particularly advantageous because it removed the ambiguity associated with model calibrations that are solely based on water level measurements. With suitable choice of the model parameters, the model reproduced the measured water level profiles of the jam very closely, while providing reasonable simulations of jam thickness. The selected values of the model parameters were consistent with values established by previous applications on other rivers, and almost identical between the two Matapedia jams, despite contrasts in jam severity and river flow. Model sensitivity was explored using the more complete 1995 data set and found to be moderate. The detail of the available data also made it possible to scrutinize hydraulic resistance characteristics of flows underneath ice jams, and confirm earlier formulations specifically designed to address extreme-roughness conditions. [Copyright &y& Elsevier]

Abstract: Since the late 1960s, a paucity of ice-jam flooding in the lower Peace River has resulted in prolonged dry periods and considerable reduction in the area covered by lakes and ponds that provide habitat for aquatic life in the Peace–Athabasca Delta region. To help identify the causes of this trend and assess future conditions under a changing climate, a field observation program was conducted during 1999–2003. Limited data were also obtained during a significant event in 2007. This program has furnished the first detailed documentation of ice processes in the lower Peace. Though focusing on the breakup period, relevant antecedent conditions were also documented. Freezeup often involves formation of thickened ice covers, which lead to high freezeup stages and potentially to enhanced growth of the solid-ice layer during the winter. Ice decay prior to breakup can be moderate to severe, and is manifested in saturation and candling. The type of observed breakup ranged from essential melt-out in 2000, to occasional jamming upstream of the delta reach in 2003, and to a moderate delta flood in 2007. The field observations and measurements, including the only ice jam profile that has been measured in the lower Peace, are shown to be consistent with current physical understanding of breakup and jamming processes. The main factors conducive to ice-jam flooding are identified and their relative importance is assessed using physically-based breakup criteria. Breakup patterns are shown to be strongly influenced by the very low slope of the river, which amplifies the importance of waves generated by ice jam releases. [Copyright &y& Elsevier]

Abstract: River ice jams can cause extreme flood events with major consequences to infrastructure, riverside communities, and aquatic life. Yet, it is only in the last few decades that concerted efforts have been made to understand and predict ice-jam occurrence and severity. Building on a 1990s state of the art review, new physical knowledge, prediction capabilities, and management methods are discussed herein. The thickness and hydraulic roughness of ice jams have been elucidated, and flow through ice-jam voids quantified. Under-ice transport of frazil ice, which can lead to enormous freezeup accumulations, has been placed on a more rigorous footing, while heat exchange with the water flowing under a jam can be predicted with some confidence. Systematic field measurements have produced new understanding of the waves that are generated by ice-jam releases. Increased understanding of the physical processes has enhanced confidence in older numerical models and motivated development of more sophisticated ones, leading to two-dimensional dynamic algorithms based on continuum as well as discrete element approaches. Ice-jam management and control continue to be difficult tasks, but new structural and non-structural techniques offer promising avenues, at least in the case of relatively small rivers. The emerging issue of climate change and the growing appreciation of related ecological linkages have led to important, but still few, insights on how ice jamming regimes can be modified by altered climatic conditions and what the repercussions might be on river ecology. Despite the progress, there are still major unknowns, particularly related to the conditions of formation and release of ice jams. [Copyright &y& Elsevier]

Abstract: The water temperature decay under ice jams that form during the breakup of the river ice cover is evaluated using different data sets from several Canadian rivers. In all cases, the water temperature varies with downstream distance in negative exponential fashion, as indicated by a simplified, one-dimensional, formulation of heat conservation in a river. The corresponding heat transfer coefficients and derivative dimensionless quantities are very high relative to the values that would be expected by extrapolation of existing formulae for turbulent convection in rough pipes. It is suggested that these high heat transfer rates are due to two factors, both characteristic of breakup ice jams: the extreme roughness of the jam underside, and the flow through the voids of the accumulation of ice blocks that comprise the jam. [Copyright &y& Elsevier]

Extreme ice-jam flood events in rivers occur during a type of breakup that is partly governed by the mechanical properties of the ice cover, and known as “mechanical”. By contrast, thermal breakups are preceded by advanced thermal decay of the ice and can only produce insignificant, if any, jamming. Identification and quantification of threshold conditions that demarcate the two types of breakup events will improve current capabilities in a variety of issues, including river ice–aquatic ecosystem interactions, socioeconomic impacts of river ice, and climate-change impacts via modified river ice regimes. Extending recent advances in the prediction of the onset of mechanical breakups, a simple threshold criterion is postulated and its quantitative consequences are worked out. It is shown that there is a site-specific rise in water level above the freeze-up elevation, which delineates mechanical from thermal events. The threshold value is approximately proportional to the thickness of the ice cover, and also depends on local river morphology and hydraulics. These predictions are tested against three extensive case studies and found to perform satisfactorily. Environmental implications of the present results are discussed. [Copyright &y& Elsevier]

Major ice jams forming in the lower reaches of Peace River during the spring breakup of the ice cover have been identified as the main agents of flooding and replenishment of the perched basins of Peace–Athabasca Delta, one of the world's largest inland freshwater deltas. The paucity of ice jamming in the lower Peace River following construction of the Bennett Dam (1968) and the potential impacts of climate change have raised concerns regarding habitat degradation. The most recent ice-jam flood, which occurred in 2014, was only the fifth in the post-regulation period and the first since 1997. Despite the sizeable number of years in the regulation period, conventional estimates of flood frequency are shown to be uncertain, but become robust when based on the slope of the cumulative number of floods. The evolution of ice breakup in 2014 was in full accord with previously postulated mechanisms for the flat reach of Peace River, which extends for several hundred kilometres above its mouth. An updated examination of historical hydrometric data at the WSC Peace Point gauge took into account the years 2003–2018, extending the record of the regulation period by 50%. The analysis further confirmed and fine-tuned physically based predictions of the effects of the freezeup level, the breakup flow, the thickness of the winter ice cover, and the accumulated degree-days of thaw on the incidence of ice-jam flooding. Commercially motivated reduction of freezeup flows in recent years likely contributed to the occurrence of the 2014 flood. Total winter snowfall at an index station appears to have reversed a pre-2000 decreasing trend, but its benefit to breakup flow may be moderated by increased incidence of mid-winter thaws. • Peace River ice jams generate vital flooding of Peace-Athabasca Delta. • Drying trend and paucity of ice jams experienced since regulation in 1968. • Ice-jam flood of 2014 first since 1997, fifth since 1968, well documented. • Observed ice breakup evolution confirms postulated mechanism. • Updated time series of hydroclimatic controls reveal new insights. [ABSTRACT FROM AUTHOR]