Photos of Pansipit River suddenly drying up in Batangas has been going around in social media since yesterday. I would like to make an intelligent guess (or hypothesis) as to why this has happened.
Based on people’s observations, the river’s depth used to be about a person’s height but now the water has diminished to the point that the river floor is exposed (photos below). It’s hard to tell where exactly along the river the photos were taken as the person/s who posted them on social media did not include details such as barangay and town. Anyway, this drying up was sent to PHIVOLCS and the agency has acknowledged those observations and might send a team there to investigate, if they haven’t already. The agency also mentioned that cracks were observed within the valley of the river.
Below is a screenshot of Pansipit River on Google Earth:
I have added the yellow pin on Google Earth to label the location of the headwater of Pansipit River. Please take note that I have never been in the area myself so I’m relying now on Google Maps and those reported by people who live in Agoncillo as posted on Facebook. The river connects Taal lake (the bigger lake with tilapia and tawilis) to Balayan Bay which means, it drains Taal Lake (upper right corner) and dumps its water + sediments (and garbage might I add) into Balayan Bay (lower left corner, deep blue).
Given the limited information I have above, plus, the Google Earth terrain map, here are my hypothesis on why the river has dried up so rapidly (in one day):
First Hypothesis: Temporary Damming at the Headwater
It is possible that with the massive ash exploded out by the volcano, the thick ash may have temporarily dammed the headwater of Pansipit River thus, lessening the water supply. If you remember, I mentioned in my previous post that when the ash is mixed with water, it hardens like cement. It is possible that the ash has temporarily cemented the headwater of the river thus, preventing water from flowing into the valley downstream. To check whether this is the case, a team of experts may need to conduct a fieldwork around the headwaters to check the change in the depth of water in Taal and to see if there is an actual wall of ash that clogged the headwaters.
Does damming pose risks on people living along the river downstream?
Yes. With the tremors going around the lake and the ash material being pretty new (or fresh), the damming can be temporary. The “ash cement wall”, if I may call it that, may eventually lose its holding power if the water becomes too thick due to ash and other debris and thus, becomes too heavy. Once the temporary dam breaks, expect flooding along the river and the towns along its banks by virtue of gravity. It is one of the reasons why cloud seeding is NOT recommended. Lahar flows and floods are the last things we want at the moment. Read about Pinatubo lahar flows to see how devastating they can get.
Second Hypothesis: The Water is Being Diverted Elsewhere Through the Fissures
It has been reported that the fissures along the Pasipit River valley are kilometers-long. Again, I’m relying on what are being reported on social media. The fissures are allegedly deep, more than a ruler deep. It is thus, possible that some of the waters are being diverted to other areas. Again, a team of experts from PHIVOLCS may need to go there in order to validate, if they haven’t done so already.
Question is, can you do something to help our heroes at PHIVOLCS collate data? You can. If the river is being diverted, they have to exit somewhere, right? If you live in Lemery, Agoncillo and other towns near the river, observe for water gushing out of your property. If there is a spring or springs that suddenly sprung up in your area, observe it. Then, take photos and if possible, locate it using your phone’s GPS or apps. Some of the apps that you can use if you’re on Android are Google Earth and UTM Geo Map.
Once you’ve taken photos and located using the apps above, please send your “data” to PHIVOLCS through their Facebook page or on their website. Your observations will help them for sure!
Third Hypothesis: It Could Be A Combination of Both
The drying up of Pansipit River may be a combination of both and this is what I believe to be the most probable hypothesis given my limited data and also the rapid drying up – it happened in one day!
The cracks developing along Pansipit River, according to PHIVOLCS, may be related to the upwelling of magma underneath Taal. It is highly probable that the river itself developed there because of old fissures from previous eruptions of the volcano. So it is just “normal” to develop more fissures along the the river’s valley as these are already “weak points” to begin with.
Finally, if you are still within the permanent 14-kilometer radius from Taal, please evacuate! PHIVOLCS has warned that all the signs are there for an imminent eruption.