Source: Geophysical Research Letters
Droughts’ effect on the navigability of rivers is well documented: During the summer and fall of 2022, barges in the Mississippi River were left stranded, raising concerns about increased shipping costs for key products. Between 2015 and 2019, barges carried an average of 400 million tons of goods along the Mississippi River system each year, including 92% of the United States’ agricultural exports.
However, a new study by Amorim et al. found that over the past 60 years, anomalously high water levels have posed a greater threat to navigability than low water levels.
High water levels can cause problems, because as water rises, river currents speed up, making it difficult for boats and barges to maneuver safely. High water levels can also overwhelm locks and dams. If locks aren’t fully operational, navigation is limited. And if currents are too strong, locks can be closed completely, leaving barges at a standstill.
The researchers examined daily river gauge data from 1963 to 2020 for 39 sites along the Mississippi River. They then grouped sites into regions and ranked regions as having average or anomalously high or low water levels compared to their respective baseline flows.
The team found that since 1963, rankings of “average” flows have become less common than anomalously high and low flows, with the middle stretch of the Mississippi losing about 21 days of average flow over the study period, or 0.37 day per year. Their analyses also suggested that high water levels, rather than low ones, were more strongly associated with decreased navigability over the study period.
By studying the length and timing of consecutive days with anomalous water levels, the researchers also found that stretches of average flow have become more likely to be interrupted by high water levels.
Despite deteriorating conditions, the total amount of cargo shipped on the Mississippi River system has remained stable over the past decade. Although half a century of technological advances has made shipping more efficient, researchers suggest it has also obscured some of the challenge rising waters pose to navigation. (Geophysical Research Letters, https://doi.org/10.1029/2023GL104619, 2023)
—Shannon Banks (@SBanksowl), Science Writer