Morphometric and Spatial Analysis of Incised River Channels on the Southern Caspian Sea Coastal Plain (Iran, Mazandaran)

River incision, which refers to the vertical lowering of the riverbed, plays a significant role in adjusting river channels. This process is driven by a range of factors, including tectonic activity, climatic variations, sea level changes, hydrological dynamics, and anthropogenic influences. This study investigates the spatial pattern and morphometric characteristics of incised river channels on the southern Caspian Sea coastal plain, focusing on Mazandaran Province, Iran. Using ALOS PALSAR digital elevation model (DEM), Google Earth imagery, field survey and GIS-based morphometric analysis, 160 cross-sections across 17 rivers were assessed. The analysis applied non-parametric statistical tests and spatial autocorrelation methods to examine incision depth patterns and their relationships with geomorphic units, tectonic structures, and sea-level changes. Results reveal a mean incision depth of 4.7 meters, with significantly deeper incisions in alluvial fans than to coastal plains. Incision depth decreases downstream, showing a strong inverse correlation with elevation. Also, no significant differences were observed in the average incision depth among the different rivers. However, tectonic proximity, especially to faults other than the Khazar Fault, did not significantly influence incision depth. Moran's I statistics indicate that the incised river channels exhibit a strong clustering pattern. The findings highlight the dominant role of Caspian Sea base-level fluctuations during the late Holocene in shaping incision patterns, while tectonic factors appear more influential near mountain fronts. These insights are critical for geomorphological understanding, river management, flood mitigation, restoration efforts and regional planning in this dynamic region.

Keywords: Incised river, Caspian Sea, Coastal plain, Mazandaran







Introduction

River incision is the process of vertically cutting into the riverbed. This process plays a crucial role in adjusting the river's vertical dynamics and substantially influences the evolution of the river's geomorphological landscapes. Numerous studies on river incision indicate that various factors, including tectonics (Lavé and Avouac, 2001; Whittaker et al, 2007; Hu et al, 2016; Zhang et al. 2018; Wu et al, 2020; Ma et al, 2023), climate (Wobus et al, 2010; Dey et al, 2016; Malatesta et al, 2018; Lu et al, 2020; Wang et al, 2024 ), sea level fluctuations (Blum and Aslan, 2006; Bowman et al, 2010; Vital et al, 2010), hydrology (Wyżga et al, 2016; Binh et al, 2021; Hosseinzadeh et al, 2025), and anthropogenic activities (Pollock et al, 2007; Martín-Vide et al, 2010; Esmaili et al, 2013; Huang et al, 2014; Aringoli et al, 2015), contribute to this process.

Tectonic forces significantly influence river incision through uplift and subsidence processes. In tectonically active regions, uplift increases river gradients, enhancing the river's erosive power and leading to deeper incision. Climatic factors, especially changes in precipitation and temperature, affect river discharge and sediment load and can influence the river incision. Tectonic movements and climate operate as independent forcing factors, and their interaction can result in either opposing or, at the very least, complex impacts on river morphology (Wang et al, 2024). Sea-level fluctuations significantly impact river incision by modifying the base level. The base level is an external controlling factor for river systems, whether for rivers flowing into oceans and seas or at a local scale, such as a lake or a larger river (Faulkner et al, 2016). During periods of sea-level fall, rivers experience an increased gradient, which enhances their erosive capacity and promotes incision.

However, incision can occur through two main mechanisms: downstream progression and upstream progression. Downstream degradation, often caused by climatic factors, typically involves a decrease in the bed material load or an increase in water discharge. In contrast, upstream degradation, usually of tectonic origin, is generally the result of a fall in base level (Fryirs and Brierley, 2012; Ma et al, 2023). Human impacts, such as dam construction, river channelization, sediment extraction, land-use changes, and others, significantly contribute to the above mechanisms in river incision processes. While there are many causes of river channel incision, the morphological effects and hazards associated with incised channels tend to be similar across various physiographic environments (Simon and Rinaldi, 2006).

Floodplains along rivers play a crucial role in dispersing the energy of flood flows. In incised rivers, where the riverbed is deep and the banks are high, flood flows with varying return periods become concentrated within the channel. This concentration increases the stream power, which can result in undercutting and erosion of the riverbanks (Brierley and Fryirs, 2005). However, concentrating the flow within the channel can be an advantage in reducing flood risk by preventing water from spreading to surrounding areas. Lowering of groundwater levels, structural damage, ecological degradation, and reduced floodplain function are among the problems that occur in incised river channels (Hosseinzadeh et al, 2025).

Given the various drivers that can influence the formation and modification of incised channels, any disturbance caused by natural or anthropogenic causes, whether rapid or very slow over a long time, can cause instability of this type of river (Simon and Rinaldi, 2006). Therefore, understanding the processes affecting incised rivers is essential for river restoration and rehabilitation planning, construction of infrastructure, and river management.

The southern coastal plains are located between the Alborz Mountains and the Caspian Sea. Therefore, they have been affected by the tectonic uplift of the Alborz Mountains (Djamour et al, 2010; Ballato et al, 2015; Rashidi et al, 2023) and Caspian Sea level changes (Lahijani et al, 2009; Kakroodi et al, 2012). Additionally, abundant rainfall, permanent and steep rivers, and widespread settlement of urban and rural areas can potentially be effective in river incision. Paluska and Degens (1992) have mentioned the cause of river channel incision in the southern plains of the Caspian Sea due to the strength of river floods, i.e., the hydrological factor. Additional studies conducted on the rivers incision in the Caspian plain in the Sefidrud Delta (Yamani and Kamrani, 2010), the Neka River in Mazandaran Province (Emadodin, 2014), and the southwestern rivers of the Caspian Sea in Gilan Province (Hajikarimi et al, 2021) revealed that the tectonic factor was dominant in the upstream areas of the plain and near the mountain front, and in the downstream areas of the river, fluctuations in the Caspian Sea water level played a more significant role.

The rivers of the southern Caspian Sea plain are often incised, making the study of river channel incision in this region particularly significant due to its distinctive environmental, geomorphological, and socio-economic challenges. Hence, this research focused on performing a spatial and statistical analysis of the incised rivers in the area.

The effects of base level changes on river systems are complex. At least ten variables play a role in them, divided into three groups: base level controls, geological factors, and geomorphological controls (Schumm, 1993). Based on these characteristics and their combination, rivers are modified by erosion, deposition, river planform changes, and channel widening and narrowing. The slope of the continental shelf is also an effective factor in these changes. The slope of the coastal plain in eastern Mazandaran is less than the slope of the shelf, so in such a case the river performs bed incision to reach a state of equilibrium. In the western Mazandaran plain, due to the extension of alluvial fans to the sea, the slope of the plain is high and is almost similar to the slope of the shelf; in such, a case, the river expands downstream but, no significant incision or deposition occurs during the period of sea level fall (Shcumm, 1993; Blum and Törnqvist, 2000). Therefore, changes in sea level cannot be simply interpreted as incision following a fall and deposition following a rise. In the case of the plains leading to the Caspian Sea, this complexity is further increased due to large, rapid, and frequent sea-level fluctuations.

The Caspian Sea level experienced significant fluctuations during the late Quaternary, ranging from +50 meters during the early Khvalynian highstand to -113 meters during the Mangyshlak lowstand (12-8 ka) (Koriche et al, 2022; Kakroodi et al, 2012). After the Mangyshlak regression, the sea level rose and fluctuated between -32 and -18 meters during the Holocene (Rychagov, 1997; Lahijani et al, 2009). Coastal landforms and associated deposits above 0 meters ASL are classified as Early Khvalynian, while those between 0 and -17 meters fall under Late Khvalynian (Dolukhanov et al, 2010). Several alluvial fans extend west of the Mazandaran Plain near the current coastline. The Haraz alluvial fan, which is the largest alluvial fan in the Mazandaran Plain, extends to a height of -10 meters. This suggests that the river deposition was dominant after the Khovlnenian transgression and during the Holocene. Therefore, the depth of the incised channels does not perfectly align with the contour lines.

The Caspian Sea level has experienced a maximum range of changes exceeding 15 meters over the past 2,200 years. The highest recorded level was -22 meters around 50 BC and AD (during the mid-Parthian period), while the lowest level reached -40 meters by the mid-Sassanid period (around 600 AD) (Leroy et al, 2022). In the eastern sections of the Mazandaran Plain, the distance between these two levels changed by 5 to 8 Km, whereas in the western part of the plain, the change was between 1 and 3 kilometers. During the Little Ice Age (LIA), from 1350 to 1850, the Caspian Sea level varied between -21 and -28 meters above sea level (Naderi Beni et al, 2013).

The study results indicate that the depth of the incised channels, both near and far from the faults, does not show a significant difference. The faults in the plain area are mainly hidden and have been identified by geophysical measurements, so no clear knickpoint indicating the effect of the fault on the local bedrock has been observed. In contrast, the situation is different for the Khazar fault, which is the most significant active fault in the region and lies at the border of two tectonic plates. According to research, the minimum uplift rate of the Khazar fault is estimated to be 2 mm per year (Nazari et al, 2021). Djamour et al. (2010) reported the uplift rate of the Khazar fault to be ~6 mm/year in the western part and 2-3 mm/year in the eastern part, indicating different fault behavior in these two segments. However, no significant difference in the depth of the river incision channels was observed in the eastern and western parts of the Mazandaran Plain. The uplift of river terraces and asymmetric terraces are among the effects of this fault on the border between the mountains and the plain. Using dating techniques for river terraces can effectively aid in reconstructing the Quaternary environment of this plain. Although the plain area is subsiding relative to the Alborz Mountains, no evidence of tilting of sedimentary layers has been observed in the plain area.