Answer: A spring freshet is the sudden rise in river flow during spring that is mainly due to snow and ice melt. Monitoring when this occurs each year can indicate changes to normal seasonal conditions.
Changes in the timing of the freshet may impact the water quality and survival of some species. Many species depend on the Long Island Sound for reproduction and shifts in water salinity, temperature and other chemical changes may significantly impact juvenile species at their most vulnerable stage. More research is needed to identify how these observed shifts may be impacting aquatic species and water quality during the spring in Long Island Sound.
This indicator is derived from measurements of river flow at a gauge at Thompsonville, CT (near the Massachusetts border). The indicator is the date (we use Julian days, or # of days into the year) that the total volume of water that has passed by the gauge exceeds half of the total volume for the year. The critical date is called the “winter-spring center of volume” or WSCV. While spring weather in New England is quite variable, the WSCV usually occurs in late March or early April.
The graph below uses a smoothing function called a LOESS regression to take out some of the yearly variability by computing a weighted average WSCV for each year that considers several years of data on each side of that year but weights the closest years most heavily. This provides a better picture of short- to medium-term (5–10 years or greater) patterns in the WSCV.
Although this analysis is relatively modern, this is one of the Partnership’s longest running climate indicator with the USGS Thompsonville gauge dating back to 1929 and can calculate WSCV from then until now. When looking at weather and climate, which have a great deal of annual and decade scale variability, long datasets (75–100+ years) are essential to understanding long-term patterns, making this a valuable indicator of a changing climate.
Despite year to year variability, the freshet in the Long Island Sound is about 10 days earlier than it was a hundred years ago. While the exact magnitude and timing of the freshet in any given year is dependent on local and regional weather patterns, the long-term shift towards an earlier center of volume is indicative of a general warming trend throughout the region. It is also important to note that these shifts originated in the 1960s correlating with increased temperatures. Additionally, limited snowpack may also indicate a warming trend but could impact the timing of the spring freshet very differently. This aspect needs to be investigated further.
Sign up to receive our newsletters and updates. Choose the topics that interest you most!
"*" indicates required fields