Did you know that Pasig River, the main river of Metro Manila, sometimes reverses its normal flow and instead brings saltwater from Manila Bay to Laguna de Bay? Well, during the summer months of April and May and in times of high tide, the sea level is actually higher than that of Laguna de Bay and thus the flow of water on the Pasig River goes “upstream.” Bet you didn’t know that, huh?
To prevent or lessen the increase of salinity from Manila Bay and pollution from the Pasig River itself from entering Laguna de Bay during these times of reverse flow, the Napindan Hydraulic Control Structure or NHCS was constructed in 1983. The NHCS is found on the confluence of Marikina and Pateros-Taguig rivers with Pasig River. This confluence is also the downstream endpoint of the Napindan Channel, which is the upper part of Pasig River that connects to Laguna de Bay.
Apart from preventing the reverse flow of Pasig River, the NHCS is also used for flood control. During the rainy season, most of the flooding along the Pasig River area is due to the increased water flow coming from Marikina River. The Manggahan Floodway in Pasig City was constructed to divert much of this water from Marikina River directly into Laguna de Bay. By also closing the NHCS during times of rain, the water is effectively dammed in Laguna de Bay preventing it from flooding the downstream portions of Pasig River.