The Roosevelt Island Twitterverse reports:

Image From NYC DEP According to the NYC Department Of Environmental Protection:

The majority of New York Citys sewer system is combined, which means it is used to convey both sanitary and storm flows. Sometimes, during heavy rain and snow storms, combined sewers receive higher than normal flows. Treatment plants are unable to handle flows that are more than twice their design capacity and when this occurs, a mix of excess stormwater and untreated wastewater discharges directly into the citys waterways at certain outfalls to prevent upstream flooding. This is called a combined sewer overflow (CSO). CSOs are a concern because of their effect on water quality and recreational uses in local waterways.

Here’s more on

NYC Combined Sewer Overflow, UPDATE 5/10 – The East River slimy ooze came from a Con Ed oil spill according to Gothamist.