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?
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.