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System Description
The Seneca River is a large river that drains about 8960 km2 of central New York to the Oswego River, and subsequently Lake Ontario. The average annual flow at Baldwinsville, NY (8 km upstream of the point of entry of the Onondaga Lake outflow), over the period 1951 to 1991 was 96.3 m3·s-1. Much of the river is part of the Barge Canal. The Seneca River combines with the Oneida River, at the Three River Rivers junction to form the Oswego River, which enters Lake Ontario at Oswego. The annual average flow at Oswego over the 1933 to 1991 period was 188.6 m3·s-1. The area of the Oswego River drainage basin is about 13,200 km2 (includes Seneca River). The overall system is sometimes referred to as the Three Rivers system.
The Three Rivers system has multiple uses including navigation, hydroelectric power generation, fishing, contact recreation, and waste disposal. The natural flow and mass transport characteristics of the river system have been greatly altered (e.g., dams and locks) to support navigation and hydroelectric power generation. This has reduced the river's capacity to compensate for oxygen depletion through atmospheric inputs of oxygen, and has contributed to the unusual bi-directional flow (see below) that occurs between the Seneca River and Onondaga Lake.
Three eutrophic lakes, Cross Lake, Oneida Lake, and Onondaga Lake, play important roles in influencing the water quality of this river system. Metropolitan Syracuse, Onondaga Lake and the river system are particularly tightly linked with respect to water quality, ecologic concerns and management issues.
Fish populations actively migrate between these systems, and the lake receives inflow from the river as well as discharging to the river. The water quality of the Seneca river upstream of Onondaga Lake is also an important concern because a leading remediation alternative for the domestic waste problems of the lake, diversion of an upgraded (Metro) effluent now received by the lake to the river, depends critically on the water quality and assimilative capacity (amount of waste that can be received without causing violations of water quality standards) of the river.
Water Quality Upstream of Onondaga Lake and the Zebra Mussel Invasion
Before the establishment of dense zebra mussel populations in the river (starting in 1992), the Seneca River downstream of Cross Lake and the Oswego River had low clarity, high concentrations of phytoplankton, low concentrations of available (e.g., dissolved) nutrients, and nearly saturated oxygen concentrations. These contributions reflected the highly eutrophic (e.g., hypereutrophic) conditions that prevailed in the lakes, particularly Cross Lake.
The invasion of the river system (via connections to the Great Lakes) by the zebra mussel in the early 1990s profoundly changed the water quality characteristics of the Three Rivers system. The zebra mussel (formal biological name is Dreissena polymorpha) is a small bivalve mollusk (clam; maximum shell length of ~ 3.5 cm) that is native to southern Russia. It is believed to have reached the Great Lakes in the ballast of foreign ships. This invader has been particularly successful (i.e., dense populations supported) in productive hardwater systems with large quantities of solid substrate (e.g., rocky bottom to support attachment). Dense populations of zebra mussels cause water clarity to increase, concentrations of phytoplankton to decrease, nutrient concentrations to increase, and concentration of dissolved oxygen to decrease. These changes are associated with three different functions of the invader, filter feeding, respiration, and excretion. The loss of assimilative capacity for oxygen demanding waste in the river from the zebra mussel infestation compromised the management alternative of diversion of the Metro effluent to the river to rehabilitate Onondaga Lake.
The population density of zebra mussels in the CUT, a rock channel located just downstream of Cross Lake, is perhaps the highest sustained in a river in North America. Conditions are ideal at this location, as large amounts of appropriate food (phytoplankton) are supplied by the lake, and the rock bottom represents good substrate for the zebra mussel. The mussels cause substantial changes in common measures of water quality to occur from the CUT to Baldwinsville, during intervals of low river flow common in summer. The deployment of the robotic buoys (two) in the Seneca River in 2001 (above the CUT and in Baldwinsville) is intended to demonstrate impacts of the invader on the river.
Onondaga Lake Outlet/Downstream Seneca River
An unusual bi-directional stratified flow regime is set up in the Onondaga Lake outlet to the Seneca River during periods of low flow. Often relatively dense lake surface water exits along the bottom of the outlet and river water flows into the lake in the top of the outlet. Two conditions promote this phenomenon: the lake's elevated salinity (sum of concentrations of all ionic constituents), in part reflecting the continued loading of ionic waste from soda ash production (west shore of lake, closed in 1986), and the absence of a natural hydraulic (elevation) gradient between the lake and river, brought about by channelization of the river system and lowering of the lake to support navigation. The salinity-based density stratification set up in the outlet extends out into the river.
Salinity stratification was much stronger in the river before closure of the soda ash/chlor-alkali plant on Onondaga Lake, associated with the higher salinity of Onondaga Lake, which was caused by the greater loading of ionic waste from the operating facility. The salinity stratification extends upstream in the Seneca River as well as downstream from the point of entry of the lake. The upstream movement, or salt wedge effect, is commonly observed in stratified estuaries (meeting of river mouths with the ocean), but is unique for inland rivers. The longitudinal extent of stratification in the river is greater when river flow is lower (due to less turbulence).
The salinity stratification promotes the extension of Onondaga Lake's problems into the Seneca River. Strong dissolved oxygen (DO) depletion occurs in the lower part of the river upstream, as well as downstream, of the point of entry of the lake, causing violations of the DO's standard over substantial distances in both sections of the river. The oxygen depletion in the lower stratified layer of the river (contains Onondaga Lake outflow) is attributable to its isolation from the oxygen sources of reaeration (input from the air) and photosynthesis (because of inadequate light), combined with the continued exertion of oxygen consuming processes (e.g., plankton respiration). Extension of the violation of the ammonia toxicity standard, common to the lake, out in the lower river layer has also been documented. The longitudinal extent of stratification and related impact on oxygen resources of the river was greater before closure of the soda ash facility. In low flow periods during the operation of the facility the stratification persisted 14 km downstream to the dam in Phoenix; the turbulence provided by passage over the dam broke up the stratification. Presently the stratification is broken up before the confluence with the Oneida River.
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