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| Photo: Philippine Information Agency (PIA) |
Key sources of flooding
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Heavy rainfall from the typhoon
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The typhoon brought “intense to torrential rains” across northern Luzon including Cagayan Valley.
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The rainfall overloaded many rivers and tributaries feeding into the main river systems in the region.
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For example: “In Cagayan … a flash flood in neighbouring Apayao province had caused the Chico River to burst its banks, sending nearby residents scrambling for higher ground.”
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Overflowing major river system — the Cagayan River
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Much of the water ended up flowing into the Cagayan River, which runs through Cagayan Province. That river and its many tributaries form a major drainage network for the region.
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The valley is structurally predisposed to flooding: for example, the river channel narrows in certain stretches (e.g., the “Magapit Narrows”), slowing outflow and causing water to back up.
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As reported: “More than 5,000 people were safely evacuated before the overflowing Cagayan River buried the small city of Tuguegarao about 30 kilometres away.”
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Tributary catchment from surrounding mountains & upstream basins
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Heavy rainfall in the mountain ranges around the valley causes large volumes of water (and sediment) to rush downstream into the plain. One scientist said:
“The province serves as a catch basin of water from surrounding mountain ranges… both water and sediments from the mountains cause the rivers to overflow.”
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Thus, even if the rainfall in the immediate lowland area is not extreme, upstream rainfall can lead to big downstream floods.
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Dam operations and reservoir releases (context from prior events)
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Although I did not find definitive current public reports linking a large dam release to the flooding during Uwan in Cagayan Province, historical cases show this factor has mattered in this region. For example, during Typhoon Ulysses in 2020 the Magat Dam in Isabela Province was releasing water, which then contributed to downstream flooding in Cagayan Province.
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For the current typhoon Uwan, the report from Nov 10, 2025 says: “In Cagayan, … a flash flood in neighbouring Apayao province had caused the Chico River to burst its banks.”
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So, while dam release could be a factor, the major proximate cause appears to be heavy rainfall and river/tributary overflow.
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Why the flooding was so severe in that area
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The Cagayan Valley is naturally flood-prone: large catchment area, many tributaries, gentle slope in the floodplain, bottlenecks in river channel, and sedimentation reducing capacity.
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The heavy rainfall arrived when upstream and tributary systems were likely already saturated, so the added inflow had less ability to be absorbed or delayed.
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River channel constraints (narrow sections) slow down drainage, causing water to back up and flood adjacent lowlands.
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Low-lying areas around rivers like the Cagayan, Chico and others are at immediate risk when those rivers overflow.
In short
The floodwaters in Cagayan came primarily from intense rainfall over the region (including upstream mountain and tributary systems) plus overflow of rivers (especially the Cagayan River system) that drained all that water into the province’s lowland areas. Structural and geographic factors (narrow channels, sedimentation, many tributaries) made the flooding worse.
