Home > Hydrological Information > Reports > Hydrologic Conditions Annual Report 2001
Hydrologic Conditions Annual Report 2001


The Delaware River Basin began 2001 under normal hydrologic conditions. New York City (NYC) Delaware reservoir system storage on January 1 was nearly 55 billion gallons (bg) above the median storage and storage in Blue Marsh and Beltzville reservoirs in the lower basin was at normal capacity. As of early January, streamflows at Montague and Trenton, New Jersey were above normal and groundwater in select observation wells in Pennsylvania, New Jersey, and Delaware was experiencing a seasonal upward trend.

By early spring, abundant amounts of storage rested in the snow pack above the New York City Delaware reservoirs. Measurements on April 9 indicated the snow pack had a water equivalent of 24.4 bg (more than five times the normal amount for the time of year). The melting of this snow in combination with spring rains contributed to the refill of the NYC Delaware reservoirs by mid-April.

Late-May and June brought plentiful rainfall to the Delaware Basin. In late-May, up to four inches of rain fell across many parts of the basin, ending a 27-day period of no measurable rainfall in the Philadelphia region. Stormy conditions returned to the basin June 16-17 with the arrival of Tropical Storm Allison. Rainfall averaging two- to four-inches in a 24-hour period fell throughout much of the area, with several lower basin communities in Bucks and Montgomery counties in Pennsylvania receiving ten or more inches of precipitation. Such copious amounts of rain in a short period of time resulted in several streams overflowing their banks and hundreds of properties and several bridges sustaining damage from flooding.

Rainfall was generally below normal during July and August. As streamflows declined and reservoir and groundwater levels dipped, state governments responded with requests to conserve water. On August 8, the Pennsylvania Department of Environmental Protection (PADEP) announced Lancaster, Lebanon, and Schuylkill counties in the Delaware Basin were under a drought watch due to worsening hydrologic conditions. PADEP's declaration was the first of numerous drought declarations issued during the remainder of 2001. Please refer to the attached document, Chronology of Drought in the Delaware River Basin, for a timeline of drought declarations and actions.

Below-normal rainfall, peak summer water use demands, and almost daily directed releases to meet the flow target at Montague took their toll on the NYC Delaware system. Storage in the reservoirs rapidly declined by nearly 109 bg from July through the end of September (a decline of 39 percent greater than normal for this period based on median storage values). By October 29, the Delaware River Basin Commission's (DRBC) drought watch operations began after storage in the NYC Delaware reservoirs remained below the drought watch threshold for five consecutive days. Under drought watch operations, the Delaware River flow objectives at Montague and Trenton were reduced from 1,750 cubic feet per second (cfs) to 1,655 cfs and from 3,000 cfs to 2,700 cfs, respectively. New York City's maximum diversion was reduced from the normal 800 million gallons per day (mgd) to 680 mgd.

On November 4, less than one week after drought watch operations began, the basin officially entered drought warning. In order to conserve storage in the NYC Delaware reservoirs, flow targets and diversions were once again reduced. Under drought warning operations, NYC allowable diversion was reduced to 560 mgd and New Jersey's allowable diversion, normally 100 mgd, was reduced to 70 mgd. The flow objective at Montague was lowered to 1,550 cfs.

Below-normal precipitation continued into late autumn. Only 1.32 inches of precipitation fell above Montague during the entire month of November and the combined storage in the three NYC Delaware reservoirs continued to decline by more than one billion gallons a day. By November 26, storage in Cannonsville Reservoir dropped to 3.25 bg (3.4 percent of capacity), the lowest level since the reservoir went on line in 1967.

DRBC's drought operations officially began on December 1 after the Delaware reservoir storage remained below the drought threshold for five consecutive days. Allowable diversions for NYC and New Jersey were once again reduced from 560 mgd to 520 mgd and from 70 mgd to 65 mgd, respectively. The flow objectives at Montague and Trenton also were further reduced to 1,350 cfs and 2,500 cfs, respectively. By December 15, 2001, the combined storage in the three NYC Delaware reservoirs dropped to a record low storage of 63.348 bg, or 23.4 percent of capacity. At the December 18 commission meeting, DRBC passed Drought Emergency Resolution 2001-32. This resolution gave DRBC the authority to call for releases from federal, state, and privately owned reservoirs to augment flows, protect aquatic life, and control salinity in the Delaware River and its tributaries.


The observed precipitation above Montague, New Jersey for the year 2001 was 32.30 inches, or 10.88 inches below normal. Annual observed precipitation above Trenton, New Jersey was 34.88 inches, or 9.35 inches below normal. See the attached Table 1: Precipitation at Selected Stations in the Delaware River Basin for monthly precipitation data for select basin locations.


Streamflows throughout much of the Delaware River Basin averaged below normal for nearly all of the first quarter of 2001, but improved by late-March with the return of normal precipitation. Above-average amounts of melting snow pack and spilling reservoirs also contributed to the increased streamflows. By the end of April, average streamflows for the Delaware River at Montague and Trenton, New Jersey were 44 percent and 32 percent above normal, respectively. Higher than average streamflows also occurred during the month of June as heavy rains in late-May and the remnants of Tropical Storm Allison in mid-June pushed through the basin. Average June streamflows for the Schuylkill River at Philadelphia, Pennsylvania measured 72 percent above normal and streamflows along the Delaware River and the Lehigh River averaged 20 percent to 30 percent above normal.

Streamflows declined as precipitation deficits accumulated throughout the summer and autumn months. November 2001 proved to be the worst month of the year for streamflows in the basin as flows along the Delaware at Montague and Trenton measured 64 percent and 70 percent below normal, respectively. Along the Schuylkill and Lehigh Rivers, streamflows averaged as much as 72 percent below normal.

Little improvement was made by year-end as streams throughout the basin were flowing well below average for December. Delaware River flows at Montague and Trenton ended the year with average monthly streamflows of less than half of normal. See the attached Table 2: Streamflows in the Delaware River Basin 2001 for average monthly streamflows at selected stations. Also, please refer to Figure 1: Delaware River at Montague, NJ 2001 and Figure 2: Delaware River at Trenton, NJ 2001 for annual hydrographs of these two Delaware River stations.

Reservoir Storage

Lower Basin

Both the Beltzville Reservoir (located on the Pohopoco Creek, a tributary of the Lehigh River) and the Blue Marsh Reservoir (located on the Tulpehocken Creek, a tributary of the Schuylkill River) remained at or near capacity throughout 2001. As dry conditions prevailed in the basin, releases were directed by DRBC from both of these reservoirs to maintain the Delaware River flow objective at Trenton, New Jersey. Directed releases from the lower basin reservoirs began in early-August and continued as needed through late-November. Annual directed releases totaled 0.42 bg from Blue Marsh Reservoir and 0.81 bg from Beltzville Reservoir.

In late-October, Merrill Creek Reservoir (located in Phillipsburg, New Jersey) was called upon to make releases to augment flows at Trenton and to replace evaporative losses caused by power generation. These releases are required by Merrill Creek's operating plan whenever the Delaware Basin is under drought operations based on water supply storage in the NYC Delaware reservoirs. A total of 1.24 bg was released during 27 release days from October 31 through November 26.

Upper Basin

The NYC Delaware reservoirs began 2001 with a combined storage of nearly 55 bg above normal. The abundant storage in the reservoirs reflected the previous year's rainfall surplus of over five inches above Montague, New Jersey. However, below-normal precipitation above Montague in January and February of 2001 resulted in declining storage and by mid-March, storage had dropped below the long-term median. Above-normal rainfall in March and melting snow pack in the upper basin refilled the reservoirs to their full capacity of 271 billion gallons on April 13. By April 16, the NYC Delaware reservoir storage had reached an annual peak of 276.596 bg (102.1 percent of capacity). The reservoirs spilled a total of 63 bg during a period from late-March through mid-May, with the majority of the spill (48 bg) coming from Cannonsville Reservoir.

The first directed release from the NYC Delaware reservoirs in 2001 was made on May 13 and continued until May 23. These releases were made to augment low streamflows at Montague in order to meet the streamflow objective. Directed releases were needed again in July and continued on nearly a daily basis through mid-December. The Delaware River Master directed a total of 102.6 bg of releases during 2001.

Responding to continuing rainfall deficits in the upper basin, almost daily directed releases, and seasonally high water use demands, the NYC Delaware reservoir storage began a sharp decline in July which would continue to nearly the end of 2001. Storage would ultimately decline by more than 213 bg from the annual peak storage in mid-April to a record low of 63.348 bg by mid-December. For a graphical presentation of NYC reservoir storage levels for 2001, please refer to Figure 3: New York City Delaware River Basin Storage 2001.

Ground Water

During the first months of 2001, the average groundwater levels at eight observation wells in the Pennsylvania portion of the basin were below the normal average level. Water levels in three observation wells in the coastal plain region of Delaware and New Jersey varied at the start of the year. One of these observation wells, located in Kent County, Delaware, began the year with above-average groundwater levels. The two other observation wells, one located in Cumberland County, New Jersey and the other in New Castle County, Delaware, began the year with below-average levels. All of the above mentioned wells experienced a normal, upward trend through the spring.

By summer, many wells in the basin had begun their normal seasonal downward trend as evapotranspiration rates increased and limited recharge occurred. During the summer months, the groundwater in the eight Pennsylvania observation wells remained at below-average levels. As for the coastal plain wells, the Kent County observation well remained at above-average levels as of the end of June, but was seasonally trending downward. The observation wells in Cumberland County and New Castle County remained below the average levels at the end of June and were also seasonally trending downward.

An extremely dry autumn prevented the normal period of groundwater recharge from commencing. By mid-December, groundwater in all eight of the observation wells in Pennsylvania's portion of the basin remained far below average. All eight wells displayed a downward trend that should have started to reverse at this time of the year. Six of the eight Pennsylvania wells were at drought emergency status, with four of these six at record-low levels. Water levels in all three coastal plain observation wells in Delaware and New Jersey also were below their normal levels by mid-December and displaying a downward trend not typical for the time of the year. The downward trend common to all of the above observation wells would continue through the end of 2001. See the attached Figure 4: USGS Network Wells-Pennsylvania, Figure 5: USGS Well-New Castle Co., Delaware, Figure 6: USGS Well-Kent Co., Delaware, and Figure 7: USGS Well-Cumberland Co., New Jersey for graphical presentations of groundwater levels throughout 2001.


The seven-day average of the 250 parts-per-million isochlor (salt front) was located upstream of its normal location for most of the first half of 2001. The one exception to this was a brief period during early- to mid-April when melting snow pack and heavy rains resulted in high streamflows along the Delaware River and caused the salt front to retreat downstream to below river mile 55. This location, more than six miles downstream of the normal salt front location for mid-April, was to be the furthest downstream location during 2001. The salt front remained downstream of the normal location during the summer and early autumn months.

Below-normal streamflows along the Delaware River persisted through autumn, resulting in increased chloride concentrations and an advancing salt front. By late-November, the salt front had advanced to its furthest upstream location for the year, river mile 86. This location is three miles below Philadelphia International Airport and six miles upstream of November's normal mid-month location.

Rainfall in December increased streamflows along the Delaware and the salt front gradually retreated downstream, but its location remained far above normal for the time of the year. By the end of December, the salt front was located at river mile 79, one mile above the Delaware-Pennsylvania state line and five miles above the normal location for the time of the year. See the attached Figure 8: Location of the 7-Day Average of the 250-PPM Isochlor for an overview of salt front locations along the Delaware River in 2001.

This report was prepared by the DRBC Operations Branch.