By the 1960s, large portions of the park, such as Tibbetts Brook, were being polluted by human activity; in addition, the brook now flowed into the Broadway sewer at the south end of Van Cortlandt Lake.
Pollution from upstream and the highways, and spillover of chemicals used in the golf course, killed fish in the lake.
This problem was first noticed in May 1961 when thousands of dead bass, pickerel, catfish, perch, and carp floated up at the edge of the lake.
The mass-death of fish was blamed on siltation, Three years later, fish were still being killed by siltation.
City investigators took water samples from the lake and found that they contained large amounts of weeds and sediment.
About 22,000 square feet (2,000 m²) of the lake's surface area was being lost to sedimentation every year.
In addition, an algal bloom that caused the lake to be in a low-oxygen condition, endangering plants and animals in and around the lake. "
Unpleasant odors" in the summer also reduced recreational opportunities on the lake, and these conditions combined made it virtually impossible to come into contact with the lake's water without getting sick.
By 1976, there was a moratorium on all boating activities on the lake.


The Van Cortlandt Golf Course was used as a ski slope during the winters starting in 1961, and up to 3,000 visitors would use the slopes each weekend.
By 1964, with the use of artificial snow, it was also possible to ski during warmer days.
The seasonal ski slope was closed in the late 1960s when the city decided to allow golfers to use the Van Cortlandt Course during winters.
Also in the late 1960s, the city decided to build a series of public pools on the site of the Colonial Garden, consisting of a diving, a swimming, and a wading pool.
Construction on the $1.5 million pools started in early 1969 and was completed by 1970.
Because of the swampy nature of the ground underneath it, the pools soon began to crackle and set.
By 1979, the locker rooms were heavily vandalized and the diving pool had been closed.

The city's fiscal crisis in the 1970s caused the rest of the park to fall into disrepair.
A dearth of funds exacerbated the pollution of the park.
Hands-on education programs at the park were reduced to passive observations of flora and fauna.
Elsewhere in the park, excessive foot traffic was eroding the soil in the forests.
The stock of younger, replacement trees in the old-growth forest had relatively little diversity compared to other natural forests.

In 1979, New York City Councilwoman June Eisland released a report on Van Cortlandt Park.
The report noted that pollutants from the Major Deegan Expressway were entering Van Cortlandt Lake, and that the park ecosystem was also being harmed by inadequate drainage, soil sterilants that were used on the Putnam Branch tracks, and a fungicide with 8.5% cadmium content that was being sprayed on the golf course.
A year later, a private landscaping firm estimated that it would cost $4–7 million to restore the Van Cortlandt Lake.
By this point, the lake was so dirty that a small boat could not float on it, even though the lake was 15 feet (4.6 m) deep.
Catfish were the only fish that could survive in the lake water.
The city of Yonkers eventually attributed the cause of the Van Cortlandt Lake's pollution to four storm sewers that were found to be illegally connected to Tibbetts Brook upstream.

The utter disrepair in the park prompted some informal rules at the park's golf courses.
For instance, the Los Angeles Times noted that "a player was allowed to drop his ball a club length away if it rolled up against an abandoned auto, or, in one case, a boat.
To thwart robbers, besieged golfers quit playing in traditional foursomes and instead ventured forth in football-team-sized units.
Some players added an extra club—a night stick—or tucked tear gas spray into their golf bags."
Years later, one writer recalled that dozens of the course's trees died, and "flagsticks were reduced to broken bamboo poles stuck into the ground."
Weeds overgrew the course, and golfers would wear long-sleeve
By the 1960s, large portions of the park, such as Tibbetts Brook, were being polluted by human activity; in addition, the brook now flowed into the Broadway sewer at the south end of Van Cortlandt Lake. Pollution from upstream and the highways, and spillover of chemicals used in the golf course, killed fish in the lake. This problem was first noticed in May 1961 when thousands of dead bass, pickerel, catfish, perch, and carp floated up at the edge of the lake. The mass-death of fish was blamed on siltation, Three years later, fish were still being killed by siltation. City investigators took water samples from the lake and found that they contained large amounts of weeds and sediment. About 22,000 square feet (2,000 m²) of the lake's surface area was being lost to sedimentation every year. In addition, an algal bloom that caused the lake to be in a low-oxygen condition, endangering plants and animals in and around the lake. "Unpleasant odors" in the summer also reduced recreational opportunities on the lake, and these conditions combined made it virtually impossible to come into contact with the lake's water without getting sick. By 1976, there was a moratorium on all boating activities on the lake.

The Van Cortlandt Golf Course was used as a ski slope during the winters starting in 1961, and up to 3,000 visitors would use the slopes each weekend. By 1964, with the use of artificial snow, it was also possible to ski during warmer days. The seasonal ski slope was closed in the late 1960s when the city decided to allow golfers to use the Van Cortlandt Course during winters. Also in the late 1960s, the city decided to build a series of public pools on the site of the Colonial Garden, consisting of a diving, a swimming, and a wading pool. Construction on the $1.5 million pools started in early 1969 and was completed by 1970. Because of the swampy nature of the ground underneath it, the pools soon began to crackle and set. By 1979, the locker rooms were heavily vandalized and the diving pool had been closed.
The city's fiscal crisis in the 1970s caused the rest of the park to fall into disrepair. A dearth of funds exacerbated the pollution of the park. Hands-on education programs at the park were reduced to passive observations of flora and fauna. Elsewhere in the park, excessive foot traffic was eroding the soil in the forests. The stock of younger, replacement trees in the old-growth forest had relatively little diversity compared to other natural forests. 
In 1979, New York City Councilwoman June Eisland released a report on Van Cortlandt Park. The report noted that pollutants from the Major Deegan Expressway were entering Van Cortlandt Lake, and that the park ecosystem was also being harmed by inadequate drainage, soil sterilants that were used on the Putnam Branch tracks, and a fungicide with 8.5% cadmium content that was being sprayed on the golf course. A year later, a private landscaping firm estimated that it would cost $4–7 million to restore the Van Cortlandt Lake. By this point, the lake was so dirty that a small boat could not float on it, even though the lake was 15 feet (4.6 m) deep. Catfish were the only fish that could survive in the lake water. The city of Yonkers eventually attributed the cause of the Van Cortlandt Lake's pollution to four storm sewers that were found to be illegally connected to Tibbetts Brook upstream.
The utter disrepair in the park prompted some informal rules at the park's golf courses. For instance, the Los Angeles Times noted that "a player was allowed to drop his ball a club length away if it rolled up against an abandoned auto, or, in one case, a boat. To thwart robbers, besieged golfers quit playing in traditional foursomes and instead ventured forth in football-team-sized units. Some players added an extra club—a night stick—or tucked tear gas spray into their golf bags." Years later, one writer recalled that dozens of the course's trees died, and "flagsticks were reduced to broken bamboo poles stuck into the ground." Weeds overgrew the course, and golfers would wear long-sleeve
Van Cortlandt Park