WITTKOP, BENNETT, CHORMANN AND WUNSCH 1 GEOLOGY OF THE MAY 2006 SUNCOOK RIVER AVULSION by Chad Wittkop, Department of Geology, University of Wisconsin – Eau Claire, Eau Claire, WI 54701 Derek Bennett, Rick Chormann and David Wunsch, New Hampshire Geological Survey, NH Department of Environmental Services, 29 Hazen Drive P.O. Box 95, Concord, NH 03302-0095 INTRODUCTION The May 15-16, 2006 Suncook River avulsion in Epsom, NH, was the highest profile geologic event occurring in the state since the collapse of the Old Man of the Mountain in 2003 (e.g. Zezima, 2006; for Old Man collapse see Fowler, 2005). Rivers commonly change course in the form of meander cutoffs and small-scale avulsions within floodplains of braided systems, but the Suncook avulsion was unusual because the new channel cut through an area outside the documented 100- and 500-year floodplains (National Flood Insurance Program, 1978). This unique event serves as a natural laboratory for study of the processes driving river avulsion, the creation of a new river valley, and the impact of human activity in shaping these events. This field guide examines the causes of the avulsion by combining field observation with analysis of aerial photography, GPS data, and geologic maps in a GIS environment, and documents the major features visible in the field at the avulsion site. STUDY AREA The Suncook River originates in the town of Gilmanton at the outlet of Crystal Lake, which collects the inflow of several smaller brooks and streams draining the southern flanks of the Belknap Mountains in Gilmanton and Alton. Over its 35 mile length, the Suncook River drains an area of 256-square miles in southeastern New Hampshire including portions of 16 towns in four counties. The Suncook River flows south-southwest and joins the Merrimack River at Suncook Village, approximately six miles south of Concord, NH. The May 2006 avulsion site is located in the town of Epsom, approximately 9.5 miles upstream of the confluence of the Suncook River with the Merrimack (Figure 1). Just west of the avulsion site, the Suncook formerly split into two channels—a primary (west) channel and a smaller, secondary (east) channel—forming an island (Bear Island) 0.8 miles long and 0.3 miles wide. The lower member of the Silurian Rangeley Formation, a stratified metapelite, occurs at depths of less than ten feet from the surface around the northwestern portion of Bear Island (Lyons et al., 1997; Goldsmith, 1998). Between the study site and its confluence with the Merrimack, the Suncook valley follows the trace of the strike-slip Pinnacle fault, which forms the sharp eastern boundary of the valley in the study area (Lyons et al., 1997). During late-glacial times, an arm of glacial Lake Hooksett—a large glacial lake whose level was controlled by a spillway in the Merrimack Valley just south of the Suncook confluence—extended northeast into the Suncook valley, depositing a series of coalescing coarse-grained (sand to cobble gravel) ice-contact deltas as well as finer grained (fine sand to silt) lake-bottom sediments containing rhythmic bedding (Goldsmith, 1998). Where undisturbed, coarser-grained ice-contact delta deposits form a series of hummocks with as much as 60 feet of relief above the low-lying glacial lake beds, stream terraces, alluvium, small ponds, and wetlands of the valley bottom (Figure 2). At Huckins Mill at the northern end of Bear Island, two dams at heights of 13 and 5 feet height blocked the main and secondary channels of the Suncook respectively, creating a 31-acre impoundment (New Hampshire Department of Environmental Services Dam Inventory). The dams were constructed in the late 19 th century and reconstructed in the 1930s (Orff, 2006). MAY 2006 FLOOD May 2006 was the second wettest month on record in New Hampshire. A sustained event from May 10-15 contributed up to 17 inches of precipitation in southern New Hampshire and northeastern Massachusetts (National Climatic Data Center, 2006). Though the Suncook River is not presently monitored as part of the USGS stream gauging network, highest-ever flows were recorded on 12 rivers in central and southern New Hampshire as a result 2 WITTKOP, BENNETT, CHORMANN AND WUNSCH of this precipitation event, including tributaries of the Merrimack River of similar size as the Suncook (USGS News Release, 2006). Peak flow measurements averaged 24 times normal, at or exceeding the 100-year return interval. Figure 1. Location of new and abandoned Suncook River channels. Flow is from north to south. 10-foot contours reproduced from 1967 USGS 7.5-minute Gossville quadrangle. METHODS The avulsion site and surrounding areas were inspected several times following the May 15-16 event to photograph and describe the features observed. Differential GPS data were collected on May 20, 24 and June 9, 2006 using a Trimble backpack system and supplemented with field notes, photos, and Brunton compass bearings of the new channel’s path. Following field collection, GPS data were differentially corrected against a GPS base station maintained by the NH Department of Transportation. These data were compiled into a GIS database using ArcMap 9.1 ® software. WITTKOP, BENNETT, CHORMANN AND WUNSCH 3 Figure 2. Surficial geologic map of the area around the avulsion site (redrawn from Goldsmith, 1998) showing locations of glacial lake sediments, stream terraces and alluvium, glacial till, and areas of shallow bedrock (less than 10 feet depth). GPS and field data were compared with 1-meter pixel resolution digital orthophotography aquired by the NH Department of Transportation from imagery collected in April 2005, the 1967 USGS 7.5-minute topographic map of the Gossville quadrangle, a 10-m digital elevation model (DEM) derived using tagged vector contour data digitized from the USGS topographic map, and a surficial geologic map (Goldsmith, 1998). Estimates of new channel dimensions and the volume of sediment removed were computed from measured distance and area determined using GIS. Planes of equal elevation were computed from the DEM with the Raster Calculator function in the Spatial Analyst extension of ArcMap. It is important to note that, with the exception of the high-resolution orthophotography and the GPS data, this analysis is based on best-available 1:24,000-scale data, and such data are generally not intended for site-specific studies at the scale of the events discussed here. 4 WITTKOP, BENNETT, CHORMANN AND WUNSCH Points of interest around the avulsion site—including the banks of the new channel, areas of significant erosion, and high-water marks—were located using differential GPS. High-water marks were identified from physical evidence such as erosional features, limits of silt deposition, and locations of organic debris (twigs and leaves) festooned in branches and underbrush. Maps of the new channel were drawn by comparison of these data with detailed field notes, photographs, and compass bearings. The map of the new channel was further refined after comparisons with oblique aerial photos obtained by the Army Corps of Engineers in July 2006 and January 2007, and satellite imagery obtained by NASA in summer 2006. RESULTS The Suncook River now flows through a gravel pit to the northeast of Bear Island before rejoining a portion of a preexisting secondary channel that formed the eastern boundary of the island (Figure 1; approximate location of gravel pit contained within area of interest of Figure 4). 1.97 miles of former channel were abandoned, including 1.52 miles of the primary channel that formed the western boundary of Bear Island. Aside from small pools and seeps and contribution from a small tributary, the abandoned portions of the Suncook are not expected to maintain significant year-round flow, and by mid-summer of 2006 much of the abandoned reach was completely dry. Bedrock is exposed in the abandoned channel around the northern end of Bear Island. The presence of rocky substrate is unique to this reach of the Suncook and has served to restrict downcutting. Surficial geologic maps show that shallow bedrock does not occur upstream of this area in the former channel (Goldsmith, 1998; Figure 2). The new channel is 1.03 miles long, of which 0.44 miles is newly eroded. As a consequence of the overall shortening of the Suncook’s course, the average gradient of the river increased 44%, from 16 feet per mile to 23 feet per mile. In addition, the flow of the Suncook is now concentrated into a single channel, whereas it had previously split into two channels around Bear Island. As a result, the average velocity of the river will likely increase both upstream and downstream of the avulsion site, enhancing the river’s ability to erode laterally and horizontally. By summer 2006, the new channel of the Suncook had downcut as much as 5 feet below the thalweg of the abandoned channel at the point of avulsion. Erosion and subsequent downcutting of the new channel created nearly continuous exposures of glacial Lake Hooksett bottom sediments, and in places exposed Holocene wetland sediments containing macrofossil remains and nodules of the blue mineral Vivianite (Fe 3[PO 4]2·8H 2O). Eroded wetland sediments featured a sculpted texture reminiscent of larger-scale erosional features seen in bedrock-lined river channels. A distinct high-water mark was observed along the western face and remnants of the southern face of the gravel pit that is dissected by the new channel, indicating that water pooled there to a depth of as much as 5 feet prior to the avulsion. GIS ANALYSIS A GIS analysis of high-water marks and DEM data was undertaken to evaluate hypotheses for the initiation of the new channel—namely whether the new channel initiated from the north when the Suncook River broke through its banks, or whether the new channel resulted from headward erosion after flood waters found an outlet at the southern end of the gravel pit.