Mill River (New Haven)
Drainage Basin: South Central Coast

1. About this Watershed

The Mill River is a small stream which gains its waters from precipitation falling on some 37.7 square miles of southern Connecticut from the towns of Cheshire, Prospect, North Haven, Bethany, Wallingford, Hamden and New Haven [3 p.2-i]. A large portion of this watershed, 36.4 square miles, feeds the future reservoir of Lake Whitney. This lake will once again become one of the many reservoirs utilized by the Regional Water Authority to supply the New Haven County residents with water. This issue is covered at greater length in section 5.
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1a. Map

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2. Geology

For most rivers, the geology of the riverbed as well as the geology of the surrounding watershed will greatly determine the flow pattern. For example, a riverbed with large amounts of sorted and stratified sand or rock deposits will be better able to have a more continuous and stable flow through wet and dry periods. The Mill river has a streambed that is made up of glacial till and ice deposits. These unsorted deposits cause a stream to have much higher peak and low flows since the water in the stream is not as contained by the uniform sediments [3 p. 2-6]. There is some evidence to support this in the Mill River. The USGS has installed a water-stage recorder on the Mill River 2.5 miles north of the Hamden town center. This recorder measured, in January through March of 1999, that the Mill River had an average flow of 104.2 cubic feet per second. During June through August of the same year, the average dipped to 5.51 cubic feet per second [6]. This drastic change can be attributed to the changes in evapotranspiration throughout the course of the year. The Mill River never ceases to flow in dry months however, and that is due to the saturation of the unconsolidated drift. These sediments have formed a large aquifer that is recharged during high precipitation periods and then sustains stream flow during drier periods.

The glacial deposits are a random mixture of everything from clay to some larger boulders. These ice deposits are irregular, and cause sudden changes in gradient for the stream and cause rapids and level kames or kettles to form in the flow of the river. These unsorted sediments are also easily eroded, and they form much of the sediment load that the Mill River and its tributaries transport.

Other considerations for this area is the bedrock. The Mill River is contained in the Central Connecticut Rift valley, and runs through some of the more interesting geologic areas of the state. Near the upland areas of the streambed, the underlying bedrock is schist and gneiss. The Mill River valley further down is formed from much softer New Haven Arkose Sandstones. Near the middle of the streams flow is Sleeping Giant State Park which is an intrusive dike composed of basalt. The upper zones are much more erosion resistant, and therefore create higher relief for the stream. [3 p.2-7]
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3. Hydrology

There are two areas of the river that can be helpful in understanding the hydrology of the Mill River. The first is the water-stage recorder in Hamden. This recorder only gives information for the upper 24.5 square miles of drainage area however, so the other point of informal observation for this river is the Lake Whitney dam in New Haven. At this point, 36.4 square miles of the drainage basin are accounted for. The upper gauge gives a good idea of how the stream would flow were it untouched by human uses. The dam gives a better picture of the stream after it has been used for the water needs of a community, and also is incredibly important in understanding the lower Mill River and its environmental concerns.

The water-stage recorder in Hamden has been in operation on and off from 1968. There was only one period of time that the gauge was not in use, from September 1970 till October 1978. The gauge has been in use since that time, 82.57 feet above sea level. During the last 28 years of recording, the average flows of the river have been consistent with what is normal for a stream over glacial till. The top average flow months are February-April, and the lowest flow months are from July-September. April has had the highest flow over this time period with an average flow of 93.5 cubic feet per second. September was the lowest average month with a mean flow of 13.7 cubic feet per second [6]. See diagram 2, pg 1 and pg 2.

The dam on Whitney Lake is not nearly as accurate a gauge, nor is it as easy to interpret. During periods of low precipitation, or during times of drawdown for water consumption, the lake will lose its water level, and flow over the dam will cease. Not surprisingly, the flow of the lower Mill River is drastically altered at that time. In 1974-1978, a USGS water gauge was placed on the river just after the dam. During days of zero flow over the dam, the Mill River flow dropped to 0.63 to 1.5 cubic feet per second. This represents a significant effect on the hydrology of the river [2 p. 29]. During a near- normal rain year, as in 1974, there were 15 days of zero flow over the dam. In a dry year, like in 1981 these numbers jump to over 100, and on a drought year like 1966, over 150 zero-flow days occur. This obviously drastically impacts the lower Mill River—salinity, drying of marshes and other effects can occur.

However, the dam is not the only issue that effects the hydrology of the lower Mill River. Much of the water in the lower channel is effected daily by the tides. Tidal influxes have been hindered by the creation of tidal gates near the mouth of the river. Tidal changes can cause 1-2 feet of water level rise next to these gates, and the effect of this can sometimes be seen at the foot bridge 200 feet below the Lake Whitney Dam.

There are two other dams of minor effect on the Mill River. The Axleshop dam is located north of Mount Carmel Ave. in Sleeping Giant State Park, but is partially breached. The impoundment of the five foot high masonry dam is almost full of sediment, and therefore has little effect on the peak flow rates or sediment loads. Clark’s dam is next to the New Haven Bridge in Hamden. It is also five feet high, and was rebuilt in 1986 to fix a breach and restore volume.

Average precipitation for the Mill River watershed for the past 15 years is about 49 inches/yean The runoff coefficient is also what would be expected for a temperate climate—almost 52% of precipitation becomes stream runoff. Only half of the water that precipitates on this area is evaporated again before it can become runoff This part of New England is often humid, with saturated air that cannot hold any more water vapor.

There are four major tributaries of the Mill River, and numerous other gullies and riviets that fill the stream. The largest is Willow Brook which drains 12.7 square miles of area into Mill River. Butterworth Brook, Jepp Brook, Eatons Brook and Shepards Brook all drain between 2-3 square miles of area. These tributaries are obviously much smaller then the Mill River itself, and the banks of these streams are eroding. Much of the sediment load in the Mill River can be attributed to these tributaries. At least 4 of these tributaries are being targeted by the Regional Water Authority for conservation efforts. The plan is to decrease sediment erosion in these beds to increase water quality further down the river.
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4. History

The mill river is rich in cultural and economic history. One of the major concerns for the river is how the water treatment facility will effect the river and its continued aesthetic and historic importance. The Mill River obviously got its name from the Mills that were built on its banks that utilized the running water to power machinery. The first known Mill was built by the City of New Haven and sold to Christopher Todd in 1686. This mill was a grist mill and was tremendously important to the economy of the area. The many farmers were able to rent time at the mill to grind their grain and sell/use it for their needs. When this mill broke down, the farmers would have to travel to West Haven for the next closest mill—a distance that could take some Mount Carmel habitants several days in a round-trip. Other minor mills were also established during the 17 and 18 centuries—a carriage factory, a mill to make carriage wheels and axles, a brass factory, a mill for fulling cloth and a paper mill. Other minor establishments also utilized the river for hydropower [1 p. 111].

In 1798, this mill became even more prominent in the areas history when Eli Whitney bought it and the grounds to erect his arms factory. It was the first such factory in the US, and greatly increased the worth of New haven to the country. Whitney increased the height of the dam from 6 feet to about 27 feet in 1860, and 31 feet in 1866. In 1916, it was raised another 19 inches to the current height. The original dam was wooden, but Whitney Sr. and his son Eli Whitney Jr. changed the dam to masonry, with many advancements for increased utility and safety [1 p. 99]. From 1860 till 1991, the Lake created by the dam was used as a public water supply. In 1906, a slow sand filtration plant was built on Armory Street to treat the water for New Haven use. The plant was in operation till 1991, when the entire reservoir was taken offline due to the deterioration of the water treatment plant. The Mill River has continued to add to the public water supply even since 1991 due to four wells in Hamden and Cheshire that use the stratified-drift aquifer in the upper Mill River basin [2 p. 15].

The historic site of the Eli Whitney armory is an important cultural and educational site today. The Eli Whitney Museum houses much of the former plant, and many of the original buildings and tools are on display in the museum. Numerous school trips and educational facilities are also housed at this site, and thousands of visitors pass through the site and thousands more cross the historic covered bridge into East Rock Park. The site is, and will continue to be an important cultural site. One of the major concerns for this river is to be sure that these environments and historic sites are preserved for the future.
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5. Human Use & Impact

The Mill River has been, and will continue to be an important river in New Haven County. From 1962 until 1991 Lake Whitney was used as a water source for the city. The river itself has been used for energy purposes for even longer. With the creation of a new water treatment facility, the river will once again become an integral part of the Regional Water Authority’s (RWA) supply. The Mill River is strictly regulated for this reason, and all industry has been removed from the river above the reservoir. Flora and fauna have thrived in this situation, and the water quality has remained pure. However, the river abuts a major metropolitan area and is subjected to numerous non-point source pollution.

For almost 130 years, Lake Whitney provided a sizeable amount of water to the water supply of New Haven County. Some 15 mgd (millions of gallons per day) are safely extractable from the reservoir according to the RWA. Taking water from the reservoir has obvious impacts on the lake itself and on the stream downriver. Using the water in the reservoir increases the number of days where zero flow occurs over the dam. During a near normal rain year with no water being withdrawn from the reservoir, there are zero days with no flow over the dam. When the reservoir again becomes a part of the RWA water supply, a near normal year will have a projected 24 days of no overflow. This will increase with drier years. These low flow days have a direct effect on the lower Mill River corridor. There are tidal flood gates near the mouth of the Mill River, but with less flow (between 0.63 cfs and 1.5 cfs when no water flows over the dam) [2 p. 29] there will still be saline water that travels upstream. This increased salinity will have a major impact on the freshwater marshes and swamps in the lower Mill River. Many of the protected plant species will have to migrate further up river or face extinction along the Mill River.

Lowering the water level in the reservoir will also have a major effect on the lake itself. The water level in the lake is expected to drop 5 feet below the top of the dam during dry summer use. This will also drastically drop the water level along the lake shore. Lake sediments and biotic material at the bottom of the lake will become exposed to air and may become airborne, increasing dust and illness risks for the surrounding area. Although the lower Mill river flow concerns can be mitigated by releasing water from the reservoir through a valve, the water level in the lake can only rise when more water flows in to replace what has been withdrawn.

Non-Point Source Pollution
The Mill River has enjoyed status as a regulated public water supply watershed area for almost 150 years. There has been no direct municipal or industrial waste released into the stream. However, one of the major concerns for the river is its proximity to a major urban area. This creates a large amount of non-point source pollution. There is both natural and manmade non-point pollution sources, but the manmade sources are often more problematic and more abundant near a metropolitan area. Some of these pollution sources are visible, while others are not as easy to recognize. Litter and the like is easily visible, but pesticides and fertilizers might not be seen without water tests [3 5-1].

Natural materials that effect the Mill river tend to be from the decomposition and erosion of clay, silt, sand and larger rocks in the upper stream. These materials usually originate from streambed and bank, but can be increased by road sanding in winter, or land uses like agriculture. Most of these sediments travel downstream and settle out of the flow in Lake Whitney. Over time, this increases the height of the lake bottom, and decreases the water capacity of the reservoir. It is necessary to dredge this material on occasion to reinstate the holding capacity of the lake.

Human created non-point pollution can be physical, biological, or chemical in nature. Transportation activities, Urban Landscaping/Housekeeping practices, industrial activities, construction, non-stormwater connections and accident spills and/or illegal dumping all play a role in non-point source pollution for the Mill River. Over time, land use activities and development have led to an increased amount of these pollutants in the reservoir [3 5-2]. Dredging the lake may help in the removal of these pollutants from the drinking supply. See Common Pollutants Table for more information on the types of non-point source pollution for each activity.
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6a. Natural Environment

The Mill River has three distinct areas of flora and fauna due to the Lake Whitney dam. The upper Mill River is a swiftly flowing, shallow, rocky stream and the plants and animals in that area have adapted to that environment. Lake Whitney is an artificial lake, but has become the home of a wide variety of species in the past 150 years. The lower Mill River is effected by both the tides in New Haven harbor, as well as the tidal gates placed there to keep brackish water out of the stream. These three environments are effected by human use, and will continue to be effected as human use increases with the reopening of the reservoir.

Upper Mill River
The upper Mill River has been granted a rating of A/A by the regional water authority, and designated for drinking water purposes. This rating sets the stream apart as being very clean with no industrial or commercial use of the waters. There is no point source pollution of any kind on this part of the river. The upper Mill River is also kept unpolluted to keep the 13 mile long, one mile wide aquifer safe for human consumption. This aquifer is also the reason that the river can maintain decent flow during dry periods.

The swift, cool water of the Upper Mill River is the ideal setting for many types of plants and animals. Trout, bass and other fish have been found to use the upper Mill for spawning. The upper stream also is a good environment for water needy trees and plants to grow. The quick moving water separates this area from the Lake in that there are few still pools, so species that need this environment do not make it up past the reservoir.

Lake Whitney
Lake Whitney is a manmade lake. It was created when the dam was built for industrial purposes. However, this manmade lake has become the habitat for a wide variety of living things. From phytoplankton to large fish, and aquatic vegetation to shoreline plants, this new environment has become home to many creatures. One of the primary concerns for this river is how human use will upset these environments.

The high amounts of nutrients found in Lake Whitney, combined with slowly flowing water combine to create an ideal living situation for algae, bacteria, and zooplankton. These microorganisms in turn create the base of the food chain for much of the lake environment. Since the lake water is relatively warm, warm water fish, especially largemouth bass, bluegill and carp have made this lake their home. Areas of water cover either through water vegetation or fallen trees create ideal environments for warm water spawning. This lake also provides a home for aquatic plant life. Water lilies, water ferns and other pondweeds grow in the lake. Fortunately, for the regional water authority, the turbidity of the water helps keep the growth of these plants to a minimum. Increased water cover could adversely effect the quality of the water. The shoreline also creates the environment for freshwater marshes and/or swamps [2 p. 21-22].

Lower Mill River
The lower Mill River is one of the most exciting environments along this watershed. The introduction of the tidal gates at the mouth of the river have allowed the stream to create and maintain one of the only freshwater tidal marshes in the state. This marsh is created when freshwater floods an area on a tidal basis. The water remains fresh due to the tidal gates, and some of the flora and fauna here is rare and specific. High marsh grasses and brush are found throughout the lower Mill River, and many of these plants are found in few other places. One plant in this region is a “species of special concern” and two other species have been just recently removed from this list. This is perhaps the most important parts of the river, ecologically speaking, and is also the most endangered. The use of the lake for reservoir purposes will slow the flow of water in this part of the river, which in turn will increase salinity and decrease the number of times the marshes are inundated. Besides these tidal freshwater marshes there are floodplain forests, freshwater marshes, and red Maple swamps. All of these environments will be effected by human use of the river further up [2 p. 37-38].
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6. Flora (Plants)

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7. Fauna (Animals)

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8. Lesson Plans & Field Studies

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9. Personal Stories

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10. Community Groups

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11. Recreation

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12. Related Links

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13. Notes/Sources

1. Blake, William P. “History of the Town of Hamden, Connecticut”; Published by: Price, Lee & Co., New Haven, January, 1888.
2. Hudak, John P. et. Al. “Lake Whitney Water Treatment Plant: Environmental Evaluation” Prepared for South Central CT Regional Water Authority, New Haven, CT; January 1999.
3. “Upper Lake Whitney Management Study” CT Regional Water Authority, Published by Milone & MacBroom Inc. Cheshire, CT; 2002.
5. Winslow, C. E. A. “Health Survey of New Haven”; New Haven, CT, 1928.

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