An adequate amount of dissolved oxygen in water is essential for the good health of all aquatic life. Oxygen enters water by direct absorption from the atmosphere or by plant photosynthesis. The oxygen is used by plants and animals for respiration and by aerobic (aerobic simply means "in the presence of oxygen") bacteria which consume oxygen during the process of decomposition.

Among other factors,the amount of oxygen water can hold is dependent on temperature. The basic rule of thumb is colder water has the ability to hold higher amounts of dissolved oxygen than warmer water. During the summer months, warmer air temperatures and seasonal low flows raise the water temperature of lakes, streams, and rivers. As water temperature rises, the amount of oxygen the water can hold decreases. The presence of organic materials compounds this problem. Organic materials may be naturally occurring, such as leaves and branches, or they may originate from pollution such as stormwater runoff or poorly treated wastewater. Despite their origin, as organic materials decompose, dissolved oxygen supplies are depleted leaving less available for use by aquatic animals.

A reduction in the supply of dissolved oxygen can lead to numerous changes in an aquatic ecosystem. Decreases in dissolved oxygen can cause changes in the types and numbers of aquatic species. Species which cannot tolerate decreases in dissolved oxygen include mayfly nymphs, stonefly nymphs, caddisfly larvae and beetle larvae. As dissolved oxygen levels decline, these pollution-intolerant organisms are replaced by pollution-tolerant, undesirable species of worms and fly larvae. Limited dissolved oxygen also decreases the feeding, reproductive, and spawning activities of fish.

The exact requirements for dissolved oxygen varies by species, with warm water species requiring higher levels to survive. In general, dissolved oxygen levels should remain above 6 milligrams per liter to provide full support of aquatic life. However, levels of 4 or 5 mg/l are acceptable for brief periods.

Water temperature is an important factor in the health of fish and aquatic ecosystems in general. Water temperature naturally increases during the warmer months of the year, but human-induced activities can cause the temperature to rise higher than they naturally would. Thermal pollution is one example of such an activity. Thermal pollution consists of warm water being released into a local water body. Some sources of thermal pollution include power plant releases and storm-drain runoff which has been warmed on streets, parking lots and sidewalks.

Overdevelopment is another activity that may cause an unnatural rise in water temperature. When trees and vegetation are removed to make way for houses and businesses, exposed soil may be washed into streams and lakes. This erosion causes an increase in dissolved solids in the water. As dissolved solids increase, the water becomes turbid or cloudy. Turbity allows the absorption of the sun's rays which increases the water temperature. Loss of shade trees along streams also increases water temperature.

No matter what the cause, warmer than normal water temperature is often bad news for aquatic organisms whose bodies cannot regulate internal temperatures the way our bodies do. Warmer temperatures increase their metabolism which in turn raises their demand for oxygen. Unfortunately, since warmer water has a reduced capacity for dissolved oxygen, the increased demand for oxygen is challenged. Extended periods of warming may also lead to a change in species diversity as those species able to adapt to temperature change take the place of the intolerant species.

The salt line is not really a line at all. It is rough estimation of where along the Delaware River the 7-day chloride concentration is 250 parts per million (ppm). The major source of chlorides in the Delaware River Estuary is from the intrusion of oceanic salts. Sea water has a chloride concentration of approximately 19,000 mg/L and distance from the sea affects concentration of chlorides in the estuary. Thus, locations along the Delaware River that are closer to the sea, such as Wilmington,Delaware, will have naturally higher chloride concentrations than locations further away from the sea, such as Philadelphia, Pennsylvania. The salt line seasonally advances and retreats along the river, advancing upstream during the summer and early fall when flows in the river are lower and concentrations of chlorides higher, and retreating during late fall, winter, and spring as flows increase and flush chlorides out to sea. Chlorides also naturally advance and retreat with each tide cycle.

Chloride data is determined daily from measurements taken from a U.S. Geological Survey monitor at Reedy Island Jetty near Port Penn, Delaware, and from the Kimberly Clark Corporation in Chester, Pennsylvania.

Chloride data is not directly measured from Reedy Island. Instead, specific conductance measurements are recorded 24 times a day (specific conductance is simply the ability of a solution to conduct electricity and it is measured in units called micromhos). Since specific conductance and chlorides are directly related, a table is used to convert the specific conductance data to chloride concentrations (chloride concentrations are measured in milligrams per liter [mg/L] or parts per million [ppm], where one mg/L is equal to one ppm). As a final step, the 7-day average of chlorides from each of the reference sites is calculated and used to interpolate where on the Delaware River the 250-mg/L concentration of chlorides is located. If the 250 mg/L concentration moves upstream of Chester, monitors located at Fort Mifflin, near the mouth of the Schuylkill River, and at the Ben Franklin Bridge are used for the interpolation.

Chloride concentrations in excess of 250 mg/L are usually considered undesirable for domestic use and may corrode machinery if used for industrial purposes. The Delaware River Basin Commission's reservoir operating plans require monitoring the salt line in order to set drought flow targets for the Delaware at Trenton, New Jersey. The Delaware River supplies most of the estuary inflow. Maintaining the Trenton flow is effective in limiting salinity intrusion.

The New York City (NYC) Delaware Basin Reservoir System consists of three reservoirs which supply New York City with more than half of its water supply. The reservoirs include the Neversink Reservoir, which is located on the Neversink River, the Pepacton Reservoir, located on the East Branch of the Delaware River, and the Cannonsville Reservoir, located on the West Branch of the Delaware River (see the Reservoir Location Map). Water is "drafted" or diverted from these three reservoirs and transported via aqueduct to Rondout Reservoir, where it continues its journey to NYC.

There are two major types of releases made from the New York City (NYC) reservoirs. The first type of release is a DIRECTED RELEASE. Directed releases are made from reservoirs to maintain an adequate amount of streamflow downstream from the reservoirs. The goal of directed releases from the NYC reservoirs is to maintain a daily mean flow at Montague, New Jersey, of 1,750 cubic feet per second (during normal operations).

A second type of release is a CONSERVATION release. Conservation releases are made daily from each of the three NYC reservoirs. A conservation release is an amount of water released from a reservoir to protect aquatic life downstream in close proximity to the dams.

Directed releases also are made from lower basin reservoirs to maintain flows at Trenton, New Jersey. You can learn more about this in the following Q and A.

The lower basin reservoirs covered in the daily flow and storage report include:

• F.E. Walter Reservoir is located on the Lehigh River in Carbon and Luzerne counties, Pennsylvania (used primarily for flood control);
• Beltzville Reservoir is located on Pohopoco Creek, a tributary to the Lehigh River, near Lehighton, in Carbon and Monroe counties, Pennsylvania;
• Blue Marsh Reservoir is located on Tulpehocken Creek, a tributary to the Schuylkill River, near Reading, in Berks County, Pennsylvania; and
• Merrill Creek Reservoir is located on Merrill Creek in Warren County, New Jersey (used primarily for replacement of evaporation by power plants).

The above reservoirs serve many functions, including flood control, water supply, recreation, and low flow augmentation.

One of the numerous functions of Beltzville and Blue Marsh reservoirs is to provide low flow augmentation. This means that during periods of drought or seasonal low flow, releases are directed from the reservoirs. Directed releases are used to maintain a target flow at Trenton, New Jersey, of 3,000 cfs (during normal operations) and to control the advance of the 250 mg/L salt line. Releases may be made from Merrill Creek Reservoir during times of drought to replace power plant consumption downstream.