Search

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]

The Peace-Athabasca Delta (PAD) in northern Alberta is one of the world's largest inland freshwater deltas, home to many species of fish, mammals, and birds. Coincident with regulation that commenced in 1968, the PAD has experienced prolonged dry periods in-between floods, accompanied by reduction in the area covered by lakes and ponds that provide habitat for aquatic life. This likely resulted from reduced frequency of formation of major spring ice jams, which are the only mechanism for replenishing the higher-elevation, or "perched", basins of the PAD. The inundation effectiveness of ice jams depends on the magnitude of the prevailing river flow, but it has not been possible so far to assess whether regulation has had a positive or negative effect on breakup flows. Using a comprehensive BC Hydro data set, containing river inflows up to the W.A.C. Bennet Dam, "naturalized" breakup flows are computed; these are flows that would have occurred under natural flow conditions and are tabulated for each year of the study period (1972–2016). The rigour and utility of the adopted "lagged-flow" methodology are demonstrated via basic hydrodynamic principles and comparisons with independently generated numerical-modelling data for an early portion of the regulation period. Though there can be significant differences between naturalized and regulated flows in individual years, average flow during the first week of May, the typical time when major ice jams form, has not been affected by regulation. During the last week of April and the second week of May, regulation has respectively had a positive and negative effect on average flow. Naturalized flows exhibit practically no temporal trend in the study period (1972–2016), while regulated flows exhibit decreasing, but not statistically significant (P -value > 0.05), trends. It is shown further that the time when the regulated spring outflow begins to be curtailed can be a key factor in Peace River flow conditions near the PAD. • Peace River ice jams generate vital flooding of Peace-Athabasca Delta • Regulation effect on spring breakup flow, a relevant control, is unclear • Simple and rigorous methodology is developed to compute naturalized flows • Differences from regulated flows vary in sign and magnitude in different years • Naturalized breakup flows exhibit no temporal trend during 1972–2016 [ABSTRACT FROM AUTHOR]

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]