Guidebook for field trips in Vermont: New England Intercollegiate Geological Conference, 79th annual meeting, October 16, 17 and 18, 1987: Trips A-2; C-

Caldwell, Dibney W.

Faldetta, Sara

Hanson, George F.

English

UNH Scholars' Repository

University of New Hampshire University of New Hampshire Scholars' Repository NEIGC Trips New England Intercollegiate Geological Excursion Collection 1-1-1987 Geologic and Hydrologic Effects of a Catastrophic Flood in the Cold River, Southwestern New Hampshire Caldwell, Dibney W. Faldetta, Sara Hanson, George F. Follow this and additional works at: https://scholars.unh.edu/neigc_trips Recommended Citation Caldwell, Dibney W.; Faldetta, Sara; and Hanson, George F., "Geologic and Hydrologic Effects of a Catastrophic Flood in the Cold River, Southwestern New Hampshire" (1987). NEIGC Trips. 409. https://scholars.unh.edu/neigc_trips/409 This Text is brought to you for free and open access by the New England Intercollegiate Geological Excursion Collection at University of New Hampshire Scholars' Repository. It has been accepted for inclusion in NEIGC Trips by an authorized administrator of University of New Hampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu. \-2GEOLOGIC AND HYDROLOGIC EFFECTS OF A CATASTROPHIC FLOODIN THE COLD RIVER, SOUTHWESTERN NEW HAMPSHIRED. W. Caldwell and Sara FaldettaDepartment of Geology, Boston UniversityBoston, MassachusettsandGeorge F. HansonStar Route 1, Box 216Alstead, New HampshireINTRODUCTIONThe southwestern portion of New Hampshire is characterized by small,steep-gradient watersheds which drain into the Connecticut River. The valleysides in these watersheds are exceptionaly steep, are often composed offine-grained impermeable tilL and are subject to frequent slumps and slides.Although there are terraces of sand and gravel locally, they have littlemoderating effect on the flashy characteristics of the watersheds. Intensesummer rainstorms have produced the record runoffs in this area. One suchevent occurred in August, 1986, and the effects of this storm are the subject ofthis field trip.On August 7, 1986, about. 6 inches (15 cm) of rain fell within a 2-hourperiod in a portion of the Cold River watershed in the towns of Acworth,Alstead, and Langdon, New Hampshire (Fig. 1). This intense rainfall wasconcentrated in roughly circular area of about 30 mi (78 km ) located in themiddle third of the 100 mi (260 km ) watershed of the Cold River.GEOLOGIC EFFECTS OF THE 1986 FLOODSurface runoff from the steep hillsides of this watershed formed newgullies, widened and deepened existing streams. Numerous road were washed out(Stops 1 and 3) and at least one home was severely damaged.In one such gully, erosion exposed a complex section of saprolite overlainby till (Stop 5). The saprolite is developed on sufide-bearing andesite(Ammonoosuc volcanics) and is overlain by a deeply weathered diamicton,probably colluvium, although it does somewhat resemble till. Exposures ofsaprolite do exist in other parts of New England and Quebec, but are rareenough to be noteworthy (LaSalle et al, 1985). The concensus of opinion aboutthese saprolite exposures, especially those such as that at Stop 5 that underlietill, is that they were formed during the Tertiary Period. The clay mineralogyof the sub-till saprolites described by LaSalle et al (1985) suggested to theseauthors that they were formed under a warmer and wetter climate than eitherinterglacial or Holocerie conditions. These same authors presume that saprolitewas widespread in New England and Quebec prior to glaciation and that theknown exposures are remnants of this extensive soil cover.21A-2East AckworthSouthAckworthAlsteadFigure 1. Location of the Cold River watershed. Area of intense rainfall duringAugust, 1986 flood is roughly outlined by circle.22A-2Deposits of the 1986 floodAt the base of many hillside gullies, coarse gravel was deposited onalluvial fans on the edges of flood plains (Stop 4). The fans existed prior to the1986 flood and appear to consist largely of material similar to that deposited inthat flood. It may thus be possible that events similar to the 1986 flood wereresponsible for the formation of these faas.Coarse gravel nearly fills the channel of at least, one tributary streamnear its juncture with the Cold River (Stop 6). This gravel was graded to theflood level of the Cold River, and forms a kind of delta within the channel ofthe tributary and is one of a number of records of the stage of this flood. Thegravel in this delta was eroded from within the watershed of the tributary andthus represents a deposit that, is different from the slack-water depositsdescribed by Kochel and Baker (1982) in which sediment is carried into atributary from the trunk stream.The flood waters generated within the central portion of the Cold Riverhad major effects far downstream. At the mouth of the Cold River, floodwaters estimated to be about least 10-feet (3 m) deep, with a velocity 5 feet.(150 cm) per second, moved into the Connecticut River. Gravel clasts up to 15inches (400 mm) in intermediate diameter was deposited as a delta whichprograded nearly three-fourths of the way across the Connecticut. (Stop 8).When the flood waters lowered and the delta was exposed, the Connecticut, wasconfined in a channel about 100 feet (30 m) wide. The very high flow andassociated turbulence of this flow caused exteasive bank erosion on the Vermontside of the river.On the delta surface, a number of features indicated that the flow durngthis flood was in a direction that combined the flow from the Cold River andthe Connecticut (Fig. 2). These flow indicators include shingled gravel, orientedtrees and scour pits around the base of the trees, and gravel bedforms withcrests spaced about 8 feet (2.4 m) apart (Fig. 2). Subsequent floods in Januaryand April, 1987 have moved the trees, wiped out the bedforms, and reorientedthe clasts of the delta surface. The delta has not moved measurably dowasteam,but the distal end of the delta has been eroded, widening the channel At thiswriting (July 6) the delta has reappeared, as spring and early summer high waterhas receeded. There is a small channel between the delta and the river bank(Fig. 2), and the delta can be reached only by a knee-deep wade.HYDROLOGICAL EFFECTS OF THE 1986 FLOODAbout 2 weeks prior to the intease rainfall, an observation well had beenplaced near the bottom of a kettle in a small delta. A steep bedrock hill risesabove the delta surface. Figure 3 is the hydrograph of this well that, covers theperiod of the August, 1986 flood. As may be seen in the summary of rainfall inFigure 3, abundant rain had fallen prior to the major storm. This figure alsoshows that the the water table when the well was installed was over 40 feet(12 m) below the ground surface. Within a few weeks of the August storm,recharge had found its way from the surface to the water table and eventuallyadded about 9 feet (2.7 m) to the saturated thickness of the aquifer and asquickly the water was dissipated. The aquifer is abruptly terminated to the23N A-2Figure 2. Map of the delta deposited at mouth of Cold River during August,1986 flood (See Figure 1). Trees on delta surface and gravel bedforms (jaggededges of distal end of delta) indicate flow direction during flood. Plane tableand alidade map by D.W. Caldwell and Ed Kelly, September 20, 1986.ACWORTH. NEW HAMPSHIREWELL A-2Depth to Water TableSeptemberFigure 3. Hydrograph of observation well showing recharge during flood ofAugust, 1986. Bar graph at bottom of figure indicates rainfall during period July26 - August 8, 1986. Note the very rapid rise and recession of water table. Therecharge of the aquifer was a result of direct rainfall combined with runofffrom steep hillside that was concentrated into a kettle in a glacial lake delta.24A-2north, and groundwater is discharged in a series of contact springs along thebase of the valley wall, more than 100 feet (30 m) below the delta surface.Using the concept of Specific Yield (Sy), we may estimate the amount, of waterrecharged during the event:Sy = Vwa (1)Vawhere Sy is Specific Yield (or effective porosity), Vwa is the volume of wateradded (or released) to storage, and Va is the volume of aquifer into which thewater is added or released. This analysis conceives of a unit prism of theaquifer 1 foot (or other unit) square in crass section and with a length equal tothe change in water table elevation. Because this analysis uses a unit prism,volume and length are equal Rearranging equation 1 to solve for Vwa, andassuming a reasonable Specific Yield of 0.20,Vwa = Sy x Va (2)and Vwa = 0.2 x 9 feet = 0.2 x 2.7 mand Vwa = 1.8 feet or 21.6 inches = 548 mmThis analysis indicates that the aquifer in the vicinity of the kettlerecieved about 21 inches (548 mm) of recharge while about 6 inches (150 mm) ofrain were falling. It should be remembered that other rain preceeded andfLoowed the principal storm on August 7 (Fig. 3). The total of all the rainfall inlate July and early August was about 12 inches (305 mm). We believe the excessrecharge occurred when runoff from the steep hillside above the aquifer wasconcentrated into the kettle. This secondary recharge was not observed in otherparts of the watershed. We believe that, in situations like this one, unlesssecondary recharge from steep hillsides is considered total aquifer recharge maybe underestimated.REFERENCESKochel R.C. and Baker, V.R., 1982, Paleoflood hydrology: Science, vol 215, p.535-561.LaSalle, Pierre, DeKimpe, C.R., and Laverdiere, M.R., 1985, Sub-tUl saprolitesin southeastern Quebec and adjacent New England, in Boras, H.A.,LaSalle, P., and Thompson, W.B., eds., Late Pleistocene history ofnortheastern New England and adjacent Quebec: Geological Society ofAmerica Special Paper 197, p. 13-20.25A-2ROAD LOG, COLD RIVER FIELD TRIPField trip meeting place is on Route 12 in North Walpole, New Hampshire, about1 mile south of Bellows Falls, Vermont. From the north or south, take exit 6from Interstate 91, go south on Route 5 to Bellows Falls and follow sigas forRoute 12. Meeting place is 1/2 half mile south of Green Mountain Railwayroundhouse at traffic light. Turn right on off-ramp to Connecticut River. FromBoston area take Route 140 off Route 2 near Gardiner to Route 12. FollowRoute 12 through Keene to North Walpole. From eastern New Hampshire andfrom Maine follow Route 101 from Portmouth to Keene and then Route 12 toNorth Walpole. As parking is somewhat of a problem, we may want toconsolidate into as few vehicles as passible. We will pass this way to Montpelierthis afternoon and can retrieve vehicles left here.Mileage Road Log0.00 Leave off ramp, south on Route 12.0.20 North Walpole Gauge. Mouth of Saxon's River across Connecticut.0.50 Turn left on Route 123. Mouth of Cold River.0.70 Turn left onto Cold River Road. Glacial Lake Hitchcock deltaacross valley.1.5-1.9 Whitcomb Sand and Gravel across Cold River.2.6 STOP 1. Mouth of Great Brook. Cold River Road was washed awayin flood by large eddy in river at this point. We are nearly 3 milesdownstream from area of intense rainfall in August, 1986 storm.There was no evidence of large flows from Great Brook itself.Continue along Cold River Road.3.5 intersection with Route 123. Crass Route 123 and park on right forOptional stop.STOP 2. (Optional Stop). Drewsville Gauge, discontinued by USGSin 1978. Operated by Department of Geology, Boston Universitysince 1980. Inclined staff gauge allowed estimation of 1986 floodstage of 13.00 feet. By extrapolation of stage - discharge ratingcurve this is equivalent to about 10,000 cfs. This exceeds previousrecord stage of 12 feet, and compares with the stage of 11 feetduring April 1987 floods. Note gorge below bridge and numerousbridge abutments. Return to cars and proceed south (east) alongRoute 123.5.5 Juncture of Route 123 with Route 12A. Turn right, crass ColdRiver and bear left through village of ALstead. Note till cut instream channel on left, behind playground. This is near thedownstream (western limit) of intense rainfall and erosion duringAugust 1986 flood.6.1 Turn left at Gulf station to Route 123A.26A-26.50 STOP 3. Park on left. Noir.e gravel deposits in woodsbeyond pooL This is result of both the 1986 and 1987 floods. Walk afew hundred feet along road. Note abrasion of trees in ditch on leftwhich resulted from transport of coarse sediment in Qoodwaters.Crass Route 123 to juncture with dirt road. This road wascompletly washed away in flood, leaving a channel about 30 feetwide and 10 feet deep. The road was rebuilt and the road replacedwith a 12-inch culvert. Return to cars and proceed along Route123A.8.60 Crass Cold River, find parking on shoulder.STOP 4. Alluvial fan on edge of valley. Cobble size gravel wasdeposited during intense runoff in August, 1986. Numerous fans likethis one were also covered with gravel in this flood.9.40 Turn right on Route 123 A. Note small slide scars on left.9.50 STOP 5. Find parking along right side of road and return toclearing on left side of road with blue sap lines. Large gully cut tobedrock during August, 1986 storm. Exposure on east side of gullynear large maple tree shows the following stratigraphy:3 feet laminated grey till2 feet reddish-brown diamicton, probably colluvium1 foot yellow -brown saprolitebedrock (Ammonoasic volcanias)Other exposures of saprolite along gully channel up the hill.Return to vehicles and continue along Route 123 A.10.00 Turn right. Home of Dr. and Mrs. George Haason. We will havelunch here and also visit Stop 6. Pull past house and barn and parkin indicated area.Stop 6 is within walking distance of the Haason house. Walk alongpath to Cold River and along bank in upstream direction to juncturewith Millikeas Brook.STOP 6. Cobble and boulder gravel was deposited by MillikenBrook in its own channel as the flow was slowed by flooding in theCold River. This channel deposit was graded to the level of theflood stage of the Cold River during August, 1986 flood. Thisdeposit is in effect an in-channel delta. This stream now flowsbetween coarse channel deposit. During spring flood in 1987, thechannel was so diminished in capacity that water was diverted ontoflood plain.10.01 Return to Haason's for afternoon stops (2). Return to 123A, turnright.11.4 Turn right and cross one way bridge.12.0 Park in lot of Acworth highway garage. Washed out road on leftleads to Beryl Mountain pegmatite quarry. Continue along road anddescend dirt road on right.STOP 7. Washover fan from steep slopes of Beryl Mtn. covers roadand continues to floor of kettle on right. Observation well beyond27A-2fan experienced rise in water table of about. 9 feet within a. fewweeks of the flood. Analysis of the hydrograph of this well (seeFig. 2) and assuming a specific yield of 0.2, indicates as much as1.8 feet of water were recharged into the aquifer from the kettle.This suggests that water from the steep slopes above wasconcentrated into the kettle and this water plus the direct rainfallaccounts for the high recharge.Return to vehicles. Continue ahead on road.12.2 Acworth dump. Uncoilapsed delta surface exposed in rear. Bedrockon left side of road.13.0 Turn right at mineral shop.14.2 Turn left on Route 123 A17.0 Juncture with Route 123 and Route 12 A. Straight ahead.17.8 Turn left on Route 123.19.6 Turn left at blinking light on Route 123.19.9 Lake Hitchcock delta surface.20.3 Pits on right expose forset beds in Hitchcock delta.21.2 Descend foreset slope of Hitchcock delta.21.8 Turn right.22.0 Cross Cold River.22.3 Turn right on Route 12 and almost immediately turn left ontograssy area beside road. Be careful of crossing Route 12.Walk south along roadbed of abondoned railroad. Upon reachingbridge abutment, descend slope to right. Careful of poinson ivy.Reach bank of Connecticut River.STOP 8. Delta in Connecticut River was deposited from coarsesediment carried down the Cold River during the flood of August,1986. "Paleo" flow indicators included oriented trees and gravelbedforms, as well as shingled gravel (Fig. 2). Debris in fallen treesindicated water in Cold River was about 10 feet deep when deltawas deposited. Subsequent floods in January, 1987 and ApriL 1987have eroded the distal end of the delta, but the delta has stayedpretty where as it was formed. The channel between delta andVermont side of river has been widened by erosion on both sides.Soon after delta was formed, this channel was about 100 feet wide.Return to vehicles.On Friday trip: turn north (left) on Route 12. At second light, turnleft over Connecticut River, then right and find Route 5. FollowRoute 5 for about 3 miles to 1-91.28A-3Figure 1. "Probable eastern border of the Shelburne drift".According to Stewart and MacClintock (1969, p. 99), the Shelburnedrift presumably was derived by ice moving from the northeast.Note that surface-till fabrics west of the border between Win-chester and Charlestown indicate that the Shelburne drift camefrom older drift east of the border. That situation would behighly unlikely if not impossible.29

Geologic and Hydrologic Effects of a Catastrophic Flood in the Cold River, Southwestern New Hampshire

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Geologic and Hydrologic Effects of a Catastrophic Flood in the Cold River, Southwestern New Hampshire

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