Hydromorphological and Morphometric Analysis of the Comoé River Basin (Côte d'Ivoire): Implications for Runoff and Erosion Processes

 Corresponding author: COULIBALY Léréyaha Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Hydromorphological and Morphometric Analysis of the Comoé River Basin (Côte d’Ivoire): Implications for Runoff and Erosion Processes Léréyaha COULIBALY 1, *, Moussa OUEDRAOGO 1, Ismaïla OUATTARA 1, Kouadio Assemien François YAO 1, Amidou DAO 2, and Bamory KAMAGATE 2 1 Department of Mines and Reservoirs, Training and Research Unit in Geological and Mining Sciences, University of Man, Man; Côte d'Ivoire. 2 Geosciences and Environment Laboratory, Training and Research Unit in Environmental Sciences and Management, Nangui Abrogoua University, Abidjan, Côte d'Ivoire. GSC Advanced Research and Reviews, 2025, 25(01), 216-234 Publication history: Received on 09 September 2025; revised on 25 October 2025; accepted on 27 October 2025 Article DOI: https://doi.org/10.30574/gscarr.2025.25.1.0312 Abstract The Comoé River basin is one of the major hydrological units in Côte d’Ivoire, yet its hydromorphological dynamics remain insufficiently documented despite their importance for integrated water resources management. This study aims to characterize the morphometric and hydromorphological attributes of the main sub-basins of the Comoé and to assess their influence on runoff generation and erosion processes. The methodology is based on the use of an ASTER Digital Elevation Model (30 m) combined with Geographic Information System (GIS) tools to extract and analyse parameters related to basin shape, relief, and drainage network. Results indicate that the sub-basins are predominantly elongated (circularity index < 0.3), which results in relatively long concentration times and enhanced infiltration capacity. Drainage density (Dd < 1.5 km/km²) and stream frequency (F < 5) confirm the low potential for direct runoff, highlighting the predominance of infiltration processes. However, the Kafolo sub-basin, characterized by steeper slopes (global slope index = 0.54; mean channel slope = 1.43 m/km), shows higher susceptibility to erosion. By contrast, downstream sub-basins display depositional features, favoured by nearly horizontal longitudinal profiles (“lentic flats”). This study underlines the key role of morphometric parameters in shaping the hydrological and erosional response of the Comoé basin. It provides valuable insights for hydrological planning, erosion risk mitigation, and sustainable water resource management in the face of climate variability and increasing anthropogenic pressures. Keywords: Watershed; Morphometry; Infiltration; Erosion; Drainage; Comoé; Côte d’Ivoire 1. Introduction The watershed constitutes a fundamental hydrological unit, playing a central role in the collection of precipitation and its transformation into runoff at the outlet. This transformation depends not only on climatic conditions but also on the basin’s intrinsic characteristics, including its shape, relief, hydrographic network, soil properties, and vegetation cover. The study of morphometric and hydromorphological parameters therefore provides a better understanding of flow generation processes and allows for the assessment of the system’s vulnerability to hydrological and geomorphological hazards [1], [2]. GSC Advanced Research and Reviews, 2025, 25(01), 216-234 217 In recent years, several studies have demonstrated that geomorphology and drainage indices are reliable indicators of hydrological dynamics, particularly in contexts of climatic variability and increasing anthropogenic pressure [3], [4]. Advances in the use of digital elevation models (DEMs) and geographic information systems (GIS) have enhanced the ability to extract and analyse these parameters at various spatial scales ([5], [6]). Within this framework, the morphometric and hydromorphological analysis of the Comoé Basin in Côte d’Ivoire is of particular relevance. Indeed, this basin - one of the largest in the country - occupies a strategic position within the national hydrographic network, encompassing multiple climatic zones ranging from the northern savannas to the southeastern forest formations. The Comoé River, approximately 800 km in length, represents a vital water resource for agricultural, domestic, and energy activities ([7], [8]). It feeds several hydroelectric installations and supports agricultural productivity in the surrounding regions. Ecologically, the basin hosts the Comoé National Park, a UNESCO World Heritage Site, recognized for its exceptional biodiversity and its crucial role in regional hydrological regulation ([9], [10]). However, the area is increasingly subject to pressures arising from climatic variability, deforestation, and the intensification of human activities ([11], [12]). These changes alter hydrological dynamics and exacerbate risks of erosion, soil fertility loss, and ecological imbalance. Consequently, a thorough understanding of the basin’s morphometric structure, drainage network, and hydrological response is essential to support the integrated and sustainable management of water resources [3], [4], [13]. The objective of this study is therefore to characterize the main morphometric and hydromorphological attributes of the Comoé Basin, in order to assess their influence on runoff processes, sediment dynamics, and the system’s vulnerability to hydrological hazards. 2. Material and methods 2.1. Area 2.1.1. Location The Comoé River Basin (Figure 1) represents one of the largest hydrographic systems in West Africa, with an estimated surface area of approximately 78,000 km² [14], [15]. It originates from the sandstone plateaus of Banfora in southwestern Burkina Faso, at an average altitude of around 600 m, before flowing southward across Côte d’Ivoire and discharging into the Gulf of Guinea near Grand-Bassam [16]. With a total length of about 1,160 km, the Comoé River sustains a dense and highly branched hydrographic network, which includes several major tributaries such as the Léraba, Iringa, Kongo, and Kodjoboué rivers [7], [8]. The basin spans a mosaic of contrasting climatic zones: a tropical Sudanian-type climate in the north and a humid sub-equatorial climate in the south [11]. This climatic gradient promotes significant ecological and hydrological diversity, reflected in the spatial variability of streamflows, vegetation, and soil types [13]. Ecologically, the basin encompasses the Comoé National Park, listed as a UNESCO World Heritage Site since 1983, covering an area of approximately 1,148,756 hectares [9], [16]. This park is one of the largest protected areas in West Africa, recognized for its outstanding biodiversity and its crucial role in regional hydrological regulation [10]. Furthermore, the basin plays a strategic role in the socio-economic development of Côte d’Ivoire. It serves as a major water resource for agriculture, domestic supply, and hydroelectric power generation; while also supporting agricultural productivity across several regions it traverses [7], [12]. However, the area is increasingly affected by pressures associated with climatic variability, deforestation, and intensified human activities, leading to disturbances in hydrological dynamics and the gradual degradation of soils and ecosystems [3], [4][11]. GSC Advanced Research and Reviews, 2025, 25(01), 216-234 218 Figure 1 Location of the Transboundary Comoé Basin in West Africa 2.2. Relief of the Comoé Basin The relief of the Comoé River Basin is relatively uniform and shows little variation. It appears as a vast peneplain, with altitudes ranging from 0 m to 750 m. Overall, two main types of landforms dominate the basin (Figure 2): plains and plateaus. The plains, with elevations between 0 m and 200 m, are located in the southern part of the basin. They include a coastal fringe extending from 0 m to about 40 m along the lagoonal areas. This fringe consists of a succession of tertiary clay-sandy sedimentary plateaus overlying the crystalline basement. Beyond the coastal zone lies the lower and middle GSC Advanced Research and Reviews, 2025, 25(01), 216-234 219 Comoé region, where the average elevation ranges between 50 m and 200 m. The plateaus occupy most of the basin, particularly in the central and northern regions. Their elevation varies between 200 m and 500 m and includes a few isolated summits, such as lateritic hills, which sometimes exceed 600 m in altitude [17]. Figure 2 Relief of the Comoé Basin across Altitudinal Classes 2.3. Material The ASTER Digital Elevation Model (DEM) with a spatial resolution of 30 m × 30 m was used as the main data source for morphometric and hydrological analyses. Data processing and extraction of morphometric parameters - such as elevation, slope, hydrographic network, and watershed delineation - were carried out within the ArcGIS 10.2 GSC Advanced Research and Reviews, 2025, 25(01), 216-234 220 environment, using the Spatial Analyst and Hydrology toolsets. This approach enabled an accurate characterization of the basin’s topography and drainage network, which is essential for understanding its hydrological functioning. 2.4. Methods 2.4.1. DEM Pre-processing Filling the depressions (sinks) in the DEM is a prerequisite for extracting any hydrological features based on flow direction. Except in cases of lakes, endorheic systems, or karstic terrains, such depressions are considered DEM artefacts. The first step consisted in delineating the Comoé River Basin and its sub-basins, whose outlets correspond to flow measurement stations along the main course of the river. For this purpose, the ASTER DEM (30 m × 30 m) was used after a pre-processing stage through an automated procedure in ArcGIS 10.2. The D8 algorithm developed by [18] and implemented in ArcGIS 10.2 was used to fill these depressions. Subsequently, several GIS-based routines and well-established methodologies were applied to derive parameters related to the watershed and its hydrographic network. 2.4.2. Derivation of Flow Characteristics from the DEM Surface water flow always occurs in the direction of the steepest downslope. Once the flow direction of each cell is identified, the contributing cells and the number of upstream cells flowing into a given cell can be determined. This information can then be used to delineate watershed boundaries and stream networks. The following diagram illustrates the process of extracting hydrological information such as watershed boundaries and drainage networks from a Digital Elevation Model (Figure 3). Figure 3 Diagram of Hydrological Information Modelling 2.4.3. Extraction of the Hydrographic Network First, the boundaries of the main watershed were delineated, followed by those of the sub-basins corresponding to the gauging stations along the main course of the Comoé River (Kafolo, Sérébou, Akakomoékro, Aniassué, and M’Basso), as well as the hydrographic network itself. Several classifications have been proposed to characterize river networks. However, the most widely used methods are those developed by [19], [20]. Both classifications are based on the concept of a stream segment, defined as the portion of the drainage network situated either between two confluences or between a source and a confluence. The segment is termed internal in the first case and external when it originates from a source. • The [20] classification, which was adopted in this study, follows three fundamental rules: • Any stream segment without a tributary is assigned order 1 ; GSC Advanced Research and Reviews, 2025, 25(01), 216-234 221 • When two stream segments of different orders (i and j, with i > j) join, the downstream segment takes on the higher order (i) • When two stream segments of the same order (i) meet, the downstream segment becomes order (i + 1). This classification system enables the hierarchical organization of stream segments according to their level of connectivity (Figure 4). A stream reach is defined as a set of contiguous segments of the same order, arranged in the direction of flow. The order of the basin, also referred to as the magnitude of the drainage network and denoted by Ω, corresponds to the highest stream order within the basin - thus representing the order of the reach at the outlet. Figure 4 Strahler Classification (left) and Result Derived from a DEM (right) 2.5. Compactness Index (Ic) The compactness coefficient [21] represents the ratio between the perimeter of a watershed and the perimeter of a circle having the same area as that watershed (Figure 5): Ic= P 2√πA (1) P : Perimeter of the watershed; A : Radius of the equivalent circle. A compactness coefficient close to 1 indicates a rather circular watershed, which generally has a greater infiltration potential than elongated ones. Figure 5 Equivalent ellipse of a watershed used to define a compactness index [22] GSC Advanced Research and Reviews, 2025, 25(01), 216-234 222 2.5.1. Watershed Circularity (Rc) The circularity of a watershed [23] is defined as the ratio between the area of the watershed and the area of a circle having the same perimeter as the watershed: Rc =4 πA P 2 (2) Where basin circularity values approaching 1 indicate an almost circular shape [24] and a relatively gentle slope. For such basins, infiltration tends to be more uniform, and the time taken for water to reach the outlet is shorter than that of an elongated basin [25]. Basin circularity is more strongly influenced by the length, frequency, and slope of the stream segments than by the overall slope of the watershed or the configuration of the drainage network. Low, intermediate, and high circularity values are indicative of the youthful, mature, and old stages of the tributary basin life cycle, respectively [1]. 2.5.2. Characteristic Distances of the Equivalent Rectangle The applicability of the equivalent rectangle concept to a watershed is closely related to the value of the Gravelius coefficient. Indeed, for a compactness coefficient (Ic) less than or equal to 1.12, the basin exhibits a nearly circular shape, making the geometric transformation into an equivalent rectangle impractical; in such cases, the basin can be approximated as a square. Length of the equivalent rectangle. L= Kg√A 1,12 [1+√1- (1,12 Kg)2] (3) Width of the equivalent rectangle l=Kg √ A 1,12 [ 1- √ 1- ( 1,12 Kg ) 2 ] Si Kg ≥ 1, (4) 2.5.3. Relief Attributes Hypsometric Curve At the slope and basin scale, topography plays a major role in the spatial variability of soil moisture conditions. Since most meteorological and hydrological factors depend on altitude, it is useful to study the hypsometry of the watershed by elevation classes. Several pieces of information can be derived from the hypsometric curve. Mean Elevation (E) It is defined as the average ordinate of the hypsometric curve and corresponds to the ratio of the area under the curve to the total area of the basin. It can be estimated using the following relation: E =1 A∑ai( h i +h i +1 ) 2 (5) i • A: area of the basin in km²; • AI: area between two consecutive contour lines i and i+1 (km²); • Hi: elevation of contour line i (m). Mean Slope (S) The mean slope is an important characteristic in surface runoff. It provides a good indication of the travel time of direct runoff and, consequently, of the concentration time (tc). It directly influences the peak discharge during a rainfall event. The mean slope can be estimated from the hypsometric curve. GSC Advanced Research and Reviews, 2025, 25(01), 216-234 223 S =2 EL (6) Global Slope Index The classical slope index consists of relating the elevation difference between the two extreme points to the basin length (length of the equivalent rectangle). However, to avoid extreme values, the IRD (French Institut de Recherche pour le Développement) proposed a global slope index (Ig), which will be used in this study. The slope determines whether rivers are in an erosional or depositional phase. In higher areas, rivers often contribute to the erosion of the bedrock over which they flow. Conversely, in plains, rivers flow over beds where sedimentation predominates. Ig = H 5H 95 L é q (7) • H5: Elevation corresponding to 5% of the total basin area; • H95: Elevation corresponding to 95% of the total basin area; • Léq: Length of the equivalent rectangle. Limiting Slope In general, the volume available within these depressions depends on soil roughness, which can be characterized using several indices such as Random Roughness (RR), Tortuosity (T), Limiting Elevation Difference (LD), Limiting Slope (LS), and Mean Upslope Depression (MUD). In this study, the Limiting Slope (LS) was specifically employed. The LS factor, or erosive power of the catchment, is derived solely from the Digital Elevation Model (DEM). The erosive power of a water flow depends on the drained area and the flow pattern. It is proportional to the surface area of the runoff network and to the terrain slope. For a given cell, the erosive power is equal to the square root of the contributing area multiplied by the elevation difference between that cell and the upstream cell (Equation 8). Topographic irregularities or slope breaks, as well as a wide branching network, promote zones likely to be eroded or scoured by surface water flow [26]. 2.5.4. General formula LS=( λ 22.13 ) m × ( 65.41sin2θ+4.56 sin θ+0.065 ) (8) Where : • λ = slope length (m) • θ = slope angle (in radians) • m = variable slope-length exponent, depending on slope gradient 2.6. Concentration Time It is determined for each cell as the distance along the flow path from the cell to the outlet, divided by the flow velocity. This velocity was simply defined as being proportional to the local slope when the slope value exceeded 1°, and equal to 1 otherwise. The concentration time can be estimated using several empirical formulas. [27] proposed William’s formula Tc=14.6 L S-0.1 p-0.2 (9) With Tc (in minutes) representing the basin concentration time, S (km²) the basin area, L (km) the river length from the watershed divide to the outlet, and p (–) the mean river slope. GSC Advanced Research and Reviews, 2025, 25(01), 216-234 224 2.6.1. Longitudinal Profile of the Different River Sections (Elevation Drop of the Drainage Network) This corresponds to the bed profile of a watercourse along its flow axis, which makes it possible to characterize the channel slope, or more generally, the thalweg of flow. This slope tends to decrease downstream (concave profile), with different fluvial styles succeeding one another from the headwaters to the outlet. The slope controls several parameters: the flow velocity in the channel, the shape of the flood hydrographs, the velocity of the flood wave, and the river’s water depth. For a given discharge and width, a steeper river generally exhibits a higher flow velocity and consequently a shallower depth. 2.6.2. Attributes Related to the Hydrographic Network This term refers to the set of natural drainage channels, whether permanent or temporary, through which water flows as surface runoff or is discharged from groundwater, either in the form of springs or as continuous seepage along the riverbed [28]. 2.6.3. Drainage Density Drainage density is defined, for a given basin of area A, as the total length of the drainage network (various flow channels) per square kilometre. If Li denotes the length of a tributary of any order or of the main stream, then the drainage density (Dd) is expressed as: Dd=∑Li A (10) Drainage density is a measure of the degree of fluvial dissection. It is influenced by several factors, such as climatic conditions and vegetation [29], soil characteristics and bedrock properties [30], relief [31], landscape evolution processes [32], and substrate infiltration capacity [33]. In general, a low drainage density indicates the presence of permeable soils, dense vegetation cover, and gentle slopes, in contrast to a high drainage density. A well-drained basin typically exhibits a minimum drainage density of 2.5 km/km², whereas a poorly drained basin has a drainage density lower than 1.5 km/km² [19], [34], [25]. 2.6.4. Fréquence des biefs The stream frequency, as defined by [19], is the ratio between the total number of stream segments of all orders and the area of the watershed: F = ∑N A (11) and can also be calculated according to the stream order of the watershed: Fu =∑ Nu A (12) A high stream frequency (> 5) indicates significant surface runoff and a pronounced slope [34]. 2.6.5. Soil Erodibility of the Basin (Factor K) Soil erodibility depends on several physico-chemical properties, including texture, structure, permeability, and organic matter content. In general, silty and sandy-silty soils exhibit high K values, indicating strong susceptibility to erosion, whereas clayey and sandy soils tend to be less vulnerable [35]. In this study, the K factor was computed using the empirical formula proposed by [36], which relates soil granulometric and organic characteristics: K=2.1×10-4(12-OM) M1.14+3.25(S-2)+2.5(P-3) 100 (13) where: GSC Advanced Research and Reviews, 2025, 25(01), 216-234 231 the basin also exhibits this characteristic. Overall, the soils throughout the Comoé Basin are less erodible, as indicated by the low K factor values. In this context, and compared with the other sub-basins, the soils in Kafolo are the least erodible, with the lowest mean K value, whereas those in Aniassué and M’Basso are more prone to erosion. Moreover, the soils in Sérébou and Akakomoékro are moderately erodible compared with the other sub-basins studied (Table 5). Figure 10 Erodibility Factor (K) of the Main Sub-Basins of the Comoé Table 5 Statistical Distribution of K Factor Values across the Comoé Basin K-facteur Comoe Kafolo Serebou Akakom Aniassue M’Basso Min 0.00942009 0.01378347 0.01378347 0.01378347 0.01378347 0.01378347 Max 0.02229651 0.0222793 0.02229651 0.02229651 0.02229651 0.02229651 Moyenne 0.01861725 0.01805496 0.01871344 0.01870998 0.02229651 0.01871994 Ecart-type 0.00113177 0.00116248 0.00113156 0.00105871 0.00098304 0.00095302 4. Discussion The morphometric and hydrological analysis of the Comoé sub-basins reveals a strong spatial heterogeneity in physical characteristics, directly influencing hydrological behaviour, infiltration, erosion, and surface runoff. These differences result in marked contrasts between the upstream sub-basins (notably Kafolo) and those located downstream (such as M’Basso). GSC Advanced Research and Reviews, 2025, 25(01), 216-234 232 The compactness index (Ic) and circularity ratio (Rc) show that all sub-basins have elongated shapes, indicating slow runoff and a variable infiltration potential according to longitudinal position. High compactness index values (>2) denote lower hydrological responsiveness, implying longer response times to extreme rainfall events. These findings are consistent with those of [2], who demonstrated that elongated basins dissipate runoff energy more effectively and reduce the risk of flash floods. In this context, the M’Basso sub-basin, which exhibits the highest compactness index, appears to be the most vulnerable to surface water accumulation and sedimentation. The derived hypsometric curves suggest an advanced stage of geomorphological evolution, characterized by moderate relief and gentle slopes, particularly in the lower sections. This morphology implies limited erosion, as also observed by [20] for basins in the maturity phase. However, the high mean slope of the Kafolo sub-basin (Ig = 0.54) indicates a greater erosive potential in the upstream area, where the specific elevation difference (Ds = 77.75) confirms the dominance of mechanical erosion processes. These observations suggest that runoff energy progressively decreases downstream, promoting sedimentation in the lower-altitude areas (Aniassué and M’Basso). Drainage density (Dd) and stream frequency (F) indicate a weakly drained network, typical of regions with permeable soils and dense vegetation cover. The low Dd values (<0.15 km/km²) reflect predominant infiltration across the basin, highlighting the key role of vegetation in hydrological regulation [19]. However, the higher stream frequency observed in the Kafolo sub-basin suggests a more active hydrological dynamic, likely to increase surface runoff and localized erosion. The LS (Limiting Slope) and K factors display marked contrasts depending on topography and soil type. The maximum LS factor values recorded in Kafolo confirm the significant contribution of relief to the basin’s erosive energy. Conversely, the sub-basins of Sérébou, Akakomoékro, and Aniassué show lower values, indicating relatively stable soils. The results for the K factor reveal generally low erodibility, with local variations linked to soil texture and vegetation cover. These findings are consistent with [35], [36] who demonstrated that the combination of gentle slopes, dense vegetation cover, and fine-grained soils substantially reduces overall erosive potential. Overall, the Comoé basin exhibits a well-regulated morphometric structure, with stable runoff conditions and limited flash-flood risk. Nevertheless, the upstream section (Kafolo) remains more sensitive to erosion and soil loss, particularly in the event of deforestation or unsustainable agricultural practices. Conversely, the downstream zone (M’Basso) acts as an accumulation area, promoting sediment deposition and potential groundwater recharge. These hydromorphological characteristics should be integrated into basin-wide management strategies aimed at balancing soil protection, hydrological regulation, and the sustainable use of water resources. 5. Conclusion The morphometric and hydromorphological analysis of the Comoé basin highlights significant contrasts among the subbasins, reflecting a spatial organization closely linked to topography, drainage shape, and local physical conditions. The shape indices, particularly compactness and circularity, indicate that the sub-basins are generally elongated, especially downstream, where high Ic values (up to 2.41) reflect longer concentration times and more limited infiltration. In contrast, the upstream sectors, such as the Kafolo sub-basin, display steeper slopes and greater erosive potential, promoting mechanical erosion and sediment mobility. Volume and relief indices, including the hypsometric curve and specific elevation difference, confirm a decreasing altitudinal gradient from upstream to downstream, characteristic of a basin in an advanced evolutionary stage. The low mean slope (<1%) and concave hypsometric curves indicate a morphodynamically balanced system in which sedimentation locally dominates over erosion. Furthermore, the hydrographic network attributes reveal low drainage density (Dd < 1.5 km/km²) and stream frequency below 5, suggesting predominantly permeable soils and relatively high infiltration. These characteristics confer a rather regulating hydrological behaviour on the basin, although the upstream part remains more reactive to intense rainfall events. The analysis of intrinsic erosion factors, particularly LS and K, indicates generally low erosive power across the basin, though locally accentuated in steep areas, notably in the Kafolo sub-basin. The K factor values reflect moderate to low soil erodibility, confirming the resistance of the substrate and the protective effect of existing vegetation cover. Overall, these results show that the Comoé basin functions as a relatively stable hydrological system, characterized by good infiltration capacity and low susceptibility to erosion, except in some more vulnerable upstream areas. These GSC Advanced Research and Reviews, 2025, 25(01), 216-234 233 findings highlight the need for differentiated basin management, prioritizing soil conservation and the protection of upstream areas to limit hydrosedimentary degradation and preserve the ecosystem services associated with the Comoé River. Finally, this study underscores the relevance of the integrated morphometric approach, based on the use of DEM data and GIS tools (ArcGIS 10.2), for understanding hydrological and geomorphological processes in large tropical river basins. 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