The Area.- GENERAL- Ecuador is a relatively small country located between Colombia and Peru on the Pacific coast of South America (Fig. 1). The Andes Mountains run north/south and bisect the Ecuadorian lowlands into western and eastern regions. The eastern region is rainforest and forms part of the Amazonian drainage. It harbors one of the richest freshwater fish faunas in the world. The western region of Ecuador includes several drainage systems that are all much smaller than the great Amazon drainage system. There is much less precipitation in western than in eastern Ecuador, thus western Ecuador is much drier. However, precipitation varies substantially from north to south in the western region and is much greater in the north, which continues south from the wet and highly biodiverse Choco region which spans from Panama to western Colombia. Precipitation also varies seasonally due to the influence of El Nino Current, with intense rain generally falling between January and April and relatively dry conditions occurring the rest of the year. Western Ecuador is also much more disturbed than the eastern Amazon region. Much of the forest in the west has been modified for agricultural purposes and the west is the region harboring the largest number of people in the country.

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Fig 1. Satellite image of Ecuador from Google Earth

DRAINAGES- There are four major drainage systems recognized in western Ecuador (Gomez, 1989, 2001). The systems vary from being composed of a series of tributaries converging into one major river through which the entire system drains (as is the case for the Guayas drainage) to a series of isolated rivers draining independently to the Pacific, located in the same geographic region (as is the case for the Northern and Southern Coastal Drainage Systems). Besides the Andes Mountains, the Coastal Mountain Range (Cordillera Costanera- Gomez, 1989) plays an important role in separating river systems and determining flow patters in western Ecuador. It runs generally parallel to the coast from the Esmeraldas River to Guayaquil and is composed of low elevation mountains (< 800 m; Gomez, 1989).

From north to south the major drainage systems in western Ecuador are:

1. The Esmeraldas/Santiago (Cayapas) System: This is the northernmost drainage system in western Ecuador and includes the Esmeraldas and Santiago (Cayapas) Rivers in Esmeraldas Province. The Esmeraldas River is the second largest river in western Ecuador both in terms of area and water volume drained, and it is also the wettest so the rivers also drain the largest volume of water per area in the region (Table A).

Table A. Annual Water Volume and Surface Area drained for major rivers in the Esmeraldas/Santiago System. The ratio of annual water volume drained to surface area gives a measure of relative flow, with larger numbers indicating a greater volume of water per unit of surface area. Based on Gomez (1989, 2001)

River	Annual Water Volume (Hm3/year)	Surface Area (km2)	Ratio of Volume/Area
Esmeraldas	31,217	21,418	1.46
Santiago	15,467	5,919	2.61

2. The Northern Coastal System: This drainage system is composed of a relatively large number of small isolated rivers running between the Coastal Mountain Range and the Pacific Ocean. The major rivers in the system include the Chone, Portoviejo, and Zapotal Rivers in the coastal regions of Manabi and Guayas Provinces. This drainage system includes rivers running through some of the driest parts of the country (notice the drainage volume to surface area ratio for the Zapotal River for example, which is located in the Peninsula of Santa Elena; Table B) and some of the smaller rivers are most likely seasonal.

Table B. Annual Water Volume and Surface Area drained for major rivers in the Northern Coastal System. The ratio of annual water volume drained to surface area is the same as above. Based on Gomez (1989, 2001)

River	Annual Water Volume (Hm3/year)	Surface Area (km2)	Ratio of Volume/Area
Chone	1,292	2,583	0.50
Portoviejo	504	2110	0.24
Zapotal	504	7720	0.07

3. The Guayas System: This is the largest drainage system in western Ecuador. The Guayas River drainage spans an area of approximately 32,674 km² between the Coastal Mountain Range and the Andes Mountains in the provinces of Guayas and Los Rios (Gomez, 1989, 2001). The Coastal Mountain Range plays a key role in separating the Guayas drainage system from the Northern Coastal Drainage System and (together with the Andes) in funneling water south over a larger area towards the Gulf of Guayaquil. The major tributaries include the Babahoyo, Daule, Vinces, Quevedo, Yaguachi, and Congo Rivers. Guayaquil, the largest city in Ecuador, sits on the shores of the Guayas River in the interior Gulf of Guayaquil.

Table C. Annual Water Volume and Surface Area drained for major rivers in the Guayas System. The ratio of annual water volume drained to surface area is the same as above. Based on Gomez (1989, 2001)

River	Annual Water Volume (Hm3/year)	Surface Area (km2)	Ratio of Volume/Area
Guayas	36,572	32,674	1.12

4. The Southern Coastal System: This is the southernmost drainage system in western Ecuador. It is composed of a series of small isolated rivers flowing from the western slopes of the Andes to the Pacific Ocean. The major rivers in this system include the Taura, Caņar, Balao, and Jubones Rivers. Water flow in these rivers can vary tremendously in the wet and dry seasons, with several rivers being notorious for overflowing in the rainy season (Gomez, 1989).

Table D. Annual Water Volume and Surface Area drained for major rivers in the Southern Coastal System. The ratio of annual water volume drained to surface area is the same as above. Based on Gomez (1989, 2001)

River	Annual Water Volume (Hm3/year)	Surface Area (km2)	Ratio of Volume/Area
Taura	1,796	2,630	0.68
Caņar	2,268	2,462	0.92
Balao	1,890	3,470	0.54
Jubones	1,827	4,326	0.42

Click here for a Drainage Map of Western Ecuador

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Last Updated June 6, 2008