San Carlos River Basin, northeastern Costa Rica

Natural Environment

Topography and Geomorphology

The main channel of the San Carlos River has a length of 142 km and belongs to the northern Caribbean slope of the country originating in the Central Volcanic Cordillera. The San Carlos flows into the San Juan river, which ultimately drains into the Caribbean Sea. The basin has a maximum elevation of 2326 m.a.s.l. (Platanar volcano) and a minimum of 31 m.a.s.l., showing an altitude gradient of 2295 meters (Figure 2). The headwaters of the basin are characterized by steep slopes, while the lower part exhibits a fairly flat topography. Subsequently, the headwater streams have a high transport capacity, while in the lower part the rivers take on the form of meanders with increasingly low transport capacity and more deposition.

Figure 2. Topography of the San Carlos River Basin.


The southern mountainous sector of the basin, which corresponds to the headwaters and higher elevation and steeper slopes, is composed of relatively recent volcanic materials dating back to the lower Tertiary (up to 9 Mio years) and Quaternary close to the two active volcanoes in the basin. The geologic material represents lava flows, lahars, ashes, agglomerates, tubs and gaps (Rodriguez, nd). The lower elevations towards the northern part of the basin, are characterized by recent alluvial quaternary deposits and volcanic materials from the Pliocene, formed mainly by lahar deposits, lava flows, pyroclastic rocks and sliding deposits. In general, the stratigraphy of the basin is composed of intrusive volcanic rocks and surface deposits in alluvial form (Figure 3).

Figura 3. Overview of the geology (1:200000) of the San Carlos basin.


The headwaters are predominated by Andisols, which are volcanic soils that are rapidly meteorized, with a relatively high organic matter content, a sandy or sandy-loam texture and with high infiltration capacity (IMN, nd). In the lower parts, soils are partly less developed and shallow (Inceptisols) or highly developed and can be tens of meters deep (Ultisols). The latter Ultisols are highly eroded soils, abundant in iron, deep and with a clayey texture, while the Inceptisols are much younger, acidic, high in organic matter content and equally of a clayey texture (Figure 4).

Figure 4. Soils of the San Carlos river basin.

Land cover and use

Much of the basin is covered by protected primary and secondary forests, in the headwaters and the humid tropical lowlands (Figure 5). Grasslands and pastures also occupy an important part of the mid-basin area, which emerged and exist due to livestock, and were converted from forests into pastures before the logging ban introduced as a forest law in 1996 (Rodriguez, nd). Crops dominate the lowlands with a strong expansion particularly of monocultures, mainly pineapple and sugar cane. Annual cash crops such as beans, rice, maize and cassava are also produced in the San Carlos basin.

Figure 5. Land cover of the San Carlos river basin.



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