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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]