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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]

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]