Butuan

Plain

Agusan

Marsh

What is the origin of the Butuan plain ?.

What is the influence of Agusan Marsh on the geomorphology of

Butuan

Plain?

Indicators and possible driving mechanisms of shifts in the

position of Agusan

River along the Butuan

coastal plain

Fernando P. Siringan Marine Science Institute

University of the Philippines

Diliman

Fan-Delta

Alluvial Fan fan-shaped accumulation of sediment at a point along a river where there is a decrease in gradient or when a river emerges from an incised channel

Fan-Delta?

Alluvial Fans or Fan-Deltas

are characterized by a distributary

river system with one or several active channels at a time

Alluvial Fans or Fan-Deltas

Are characterized by switching of active channel or channels

and by lateral shifting of channels

Switching or shifting of channels can be…

…self driven

…or induced by land tilting due to tectonism

…caused by floods

…induced by elevated sediment load which in turn can be due to an earthquake or a heavy rainfall

Tagudin, Ilocos

Sur

Amburayan River Fan-

Delta

Vigan, Ilocos

Sur

Abra

River

Fan-Delta

Submarine canyon

Agusan

River Fan- Delta

This map suggests that the present downstream Agusan

River

and Masao River used to be both active distributary

channels of the Agusan

Fan Delta

from J. Primavera

This map also suggests that the amount of water flowing down Agusan

River used to be higher

…this implies wetter conditions than the present

The fit suggests that the positions of the shorelines and the distributary

rivers have not changed much in the past 350 years.

However, it implies switching of the active distributary

channel

Possible paleoshoreline

trace

(from Pirazzoli, 1991)

from Maeda et al., 2003

Marine Notches

Lingayen Gulf

Paleoshoreline

during the highstand

was about 16 km inland of present shoreline

Paleoshoreline

during the highest sea level stand may have been more inland but its trace may now be covered by younger fan-delta deposits.

Agusan

River Fan- Delta

Epicenters of earthquakes from 1907 to 2001 with magnitude greater than 4

Shows frequent occurrence of deep to shallow earthquakes along the eastern side of Mindanao.

The Agusan

River Basin lies in an area with annual rainfall ranging from 2000 to 4000 mm.

For the east side, most rains come during the months of October to March.

Annual Rainfall Annual Rainfall Distribution in the Distribution in the

Philippines Philippines

(from accompanying data CD in (from accompanying data CD in OhmachiOhmachi

and Roman (2002))and Roman (2002))

The watershed of Agusan River is characterized by …

•Steep gradients

•Highly fractured rocks

•Frequent large magnitude earthquakes

•High and seasonal precipitation

Occurrence of episodic, elevated sediment discharges by the rivers are likely to be frequent.

Agusan

watershed is likely to have a high sediment yield.

Switching or shifting of the Agusan

River Fan-Delta channels can be…

…self driven

…or induced by land tilting due to tectonism

…caused by floods

…induced by elevated sediment load which in turn can be due to an earthquake or a heavy rainfall

During downpours in the upstream, parts of the surface runoff and sediment load gets retained in the marsh. The marsh also slows down material transfer downstream

Agusan

Marsh serves as a buffer of flashy river discharges from upstream.

What is the influence of Agusan Marsh on the geomorphology of

Butuan

Plain?

Butuan

Plain

Agusan

Marsh

AgusanAgusan

Marsh and its Marsh and its floodplain lakes are in floodplain lakes are in the pullthe pull--apart basin of apart basin of the Philippine Fault Zonethe Philippine Fault Zone

(from (from PunongbayanPunongbayan

(2003))(2003))

The marsh is created and maintained by tectonism.

Summary and Implications

The Butuan

coastal plain is a fan-delta.

Channel switching is likely influenced by tectonism.

The Agusan

Marsh protects the Butuan

plain from the impact of flashy river discharges.

This origin implies that channel switching is a normal process within the plain.

Present-day elevated sediment loads of rivers upstream is a threat to the longevity of Agusan

Marsh.

Tectonism

created and to a large extent, maintains the Agusan

Marsh.