Water Characteristics

Extent/Size of Coastal Resources.  The Davao Gulf has a water area of 10,500 km² and a total catchment area of 5,132 km² which is derived from the various watersheds of Sarangani, Davao del Norte, Davao Oriental, Compostela Valley, and Davao City.  The average depth of the Gulf is 17 meters and volume of approximately 112x109 m³ (V. Dupra and S.V. Smith).  Its widest point is approximately 160 km while coastline is approximately 520 km., reckoned from Calian Point in Davao del Sur to Cape San Agustin in Davao Oriental (or excluding the coastline from Calian Point to the Municipality of Sarangani). Within the Gulf are the islands of Samal and Talikud in Davao del Norte, Kopiat in Davao Oriental and Lunod in Compostela Valley (MSU-Naawan, 1995). 

Hydrology. The 33 major rivers and creeks that drain into the Davao Gulf make the inner part of the Gulf estuarine in character (V. Dupra and S.V. Smith).   The water of the Gulf is oceanic in nature considering that the Pacific Equatorial current flows westward and northwest as it reaches the western boundary rim (eastern part of Mindanao) of the Pacific Ocean. The water mass is affected by the tidal force resulting from the earth’s rotation on its axis.

Water exchange is mainly tidal with relatively good surface mixing during strong monsoon winds. The northern portion is generally estuarine in character due to the massive influx of freshwater from the flood plains especially during the rainy season.

Prevailing Tidal Fluctuations. The Gulf has a unique pattern of water movement. During flood tide, water mass south of Samal Island flows southwest averaging about 0.26 to 0.42 m/sec. In the northern portion of the island, between Bassa Point and Gill Point, water mass flows northeast, then southward and swiftly veers east-northeast during the peak tide current. During a flooding event, the more saline oceanic waters move through the deeper section of the Gulf pushing the older water mass northward. Tide waves occurring ahead at the eastern section cause older water to wave towards the area of low elevation at the western side. Eddies and standing oscillations are present.

Watershed. The Davao Gulf is the final catchment for all runoffs, erosion and effluents coming from several watersheds in the region whose headwaters come from the mountain ranges of Sarangani Province in the west and south-west portion of the region, the mountain ranges of Mount Apo (the Philippines’ highest peak) in the south portion, the mountain ranges of Davao del Norte in the north and north-west portion, and mountain ranges of Compostela Valley and Davao Oriental in the eastern side. 

Thirty-three tributaries or rivers and creeks drain into the Gulf (Figure 1).  These tributaries make up 12 major watersheds and 8 micro watersheds, as defined by the River Basin Committee for the Davao Gulf Tributaries (DENR XI). 

Davao del Sur
Balutakay Watershed 
Digos Watershed
Lais Watershed
Padada-Miral Watershed
Sibulan Watershed
Davao del Norte
Tagum-Libuganon-Saug Watershed
Tuganay Watershed
Davao City
Davao River Watershed
Lipadas-Talomo Watershed
Compostela Valley
Hijo Watershed
Kingking Watershed
Davao Oriental
Sumlog Watershed
Figure 1. Twelve major watersheds of the Davao Gulf