Investigating the effects of long-term hydro-climatic trends on Midwest ice jam events

Abstract

Ice freezeup, breakup and jamming events on northern rivers can cause extensive flooding, damage infrastructure, impede navigation, and impact stream stability and the environment. Under a changing climate the ice regime will change as well, meaning increased risk and uncertainty for affected areas. The layman's perception is that climate change is causing warming that is reducing the length of the ice season, so there will be less ice and less damaging impacts from river ice. The concept of ‘less ice’ is supported by studies showing shorter ice-affected seasons. However several studies have found that in temperate regions, the effects of climate trends – increasing temperatures and precipitation – may actually exacerbate problems. In recent years, communities in the Midwest U.S. have experienced long duration ice jam events that have resulted in flooding, damages and evacuations. This study presents a first step in identifying trends in damaging ice events in the Midwest. We analyzed the ice regime of three rivers in the region to determine if trends in the hydro-climatic data support the increasing number of damaging freezeup jams observed in recent years, but no regional pattern in trends related to breakup jams were found. We found statistically increasing trends in discharge and precipitation, with no corresponding increase in winter period temperature, which provide evidence for the change in the ice regime for this region. A hindcasting analysis was used to construct a more complete record of historical ice events. This analysis revealed a statistically significant increase in frequency of freezeup jams for most sites and no regional trend in the frequency of midwinter or spring breakup jams. Some increase in the freezeup jam strength was indicated by an increasing trend in the number of freezing degree days (FDD) in the freezeup jam formation period. Communities need to shift their emergency preparedness for river ice problems from the existing focus on relatively regular, short-acting spring breakups. Future emergency management plans need to prepare to respond to the risk and impact of freezeup jams that, according to this analysis, are increasing in frequency and strength while occurring with less predictability and having longer term effects if they freeze in place.