Photos of the Long Island Sound

Research & Monitoring

Status and Trends: LISS Environmental Indicators

Spring Freshet

Year Julian plot.date
1929 103 4/12
1930 89 3/29
1931 105 4/14
1932 97 4/6
1933 106 4/15
1934 105 4/14
1935 91 3/31
1936 85 3/25
1937 107 4/16
1938 86 3/26
1939 110 4/19
1940 114 4/23
1941 94 4/3
1942 99 4/8
1943 111 4/20
1944 109 4/18
1945 95 4/4
1946 82 3/22
1947 103 4/12
1948 96 4/5
1949 84 3/24
1950 97 4/6
1951 95 4/4
1952 99 4/8
1953 91 3/31
1954 109 4/18
1955 102 4/11
1956 111 4/20
1957 86 3/26
1958 108 4/17
1959 98 4/7
1960 98 4/7
1961 106 4/15
1962 101 4/10
1963 101 4/10
1964 95 4/4
1965 100 4/9
1966 94 4/3
1967 107 4/16
1968 90 3/30
1969 109 4/18
1970 102 4/11
1971 114 4/23
1972 113 4/22
1973 84 3/24
1974 98 4/7
1975 95 4/4
1976 90 3/30
1977 93 4/2
1978 96 4/5
1979 88 3/28
1980 97 4/6
1981 73 3/13
1982 108 4/17
1983 103 4/12
1984 100 4/9
1985 86 3/26
1986 87 3/27
1987 96 4/5
1988 96 4/5
1989 108 4/17
1990 87 3/27
1991 86 3/26
1992 96 4/5
1993 102 4/11
1994 104 4/13
1995 75 3/15
1996 108 4/17
1997 99 4/8
1998 89 3/29
1999 85 3/25
2000 96 4/5
2001 107 4/16
2002 103 4/12
2003 94 4/3
2004 96 4/5
2005 98 4/7
2006 69 3/9
2007 102 4/11
2008 88 3/28
2009 91 3/31
2010 86 3/26
2011 104 4/13
2012 82 3/22
2013 96 4/5
2014 103 4/12

 

What is the Spring Freshet?

 As temperatures rise in the spring, snow and ice that have accumulated throughout Long Island Sound’s watershed, which extends far north into Vermont and New Hampshire, begins to melt. This leads to high levels of  runoff into small streams and rivers which, in turn, drain into the Connecticut River, which provides about 70% of the fresh water input into Long Island Sound, as well as other smaller rivers. This process is called the spring ‘freshet’.

Changes in the timing of the freshet may have implications for some aquatic species and human activities along the coast. Flooded fields and marshes along the river during the freshet provide critical feeding areas for migratory waterfowl. So if the freshet comes earlier, waterfowl could be impacted if they do not adjust the timing of their migration. Changes in the timing of flooding may also provide a competitive advantage to invasive plants (such as purple loosestrife and Phragmites) in the marshes since some of these species emerge earlier than the natives. In the past, these invasives were flooded in early spring and often rotted due to submergence for prolonged periods. So, if the flooding occurs earlier, the invasives (still emerging before the natives) will no longer rot in early spring and may gain a competitive advantage over natives.

What does the Spring Freshet Indicate?

By looking at 80 years of river data, scientists at the US Geological Survey and UConn have determined that the spring freshet is occurring earlier in the spring. 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, to account for leap years) that the total volume of water that has passed by the gauge exceeds half of the total 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 blue line in the graph uses a smoothing function called a LOWESS 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 gives us a better picture of short- to medium-term (5–10 years or greater)  patterns in the  WSCV.

Although this analysis is relatively modern, the USGS Thompsonville gage dates back to 1929, and we can calculate WSCV for this entire time period,  which means that this is one of our longest running climate indicators. Particularly 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, which makes this one of our most valuable indicators of changing climate.

Status

Despite large oscillations, the freshet is getting to Long Island Sound on average about 10 days earlier than it did a hundred years ago. While the exact magnitude and timing of the freshet in any given year is highly dependent on local and regional weather patterns during the late winter/early spring period, the long term shift towards an earlier center of volume is indicative of a general warming trend throughout the region.

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