Tributary Stream fan and axial river interaction in Teesta river valley within Lesser Himalaya in Sikkim and Darjeeling Himalaya, India: Response to link between climatic change and tectonic regime

Numerous tributaries stream fans present along the Teesta river within the Lesser Himalaya in Sikkim and Darjeeling Himalaya has been studied to understand the interaction and fan dynamic. These tributary stream fans are associated with the morphological features like terrace, floodplain, channel bars and erosional features like toe cut, fan entrenchment and incised terrace scarps. Few tributary streams joining Teesta river are not having fanlobes. Most of them presents fanlobes. Tributary stream without fanlobes are represented by the matures nature of morphological parameters like hypsometric curved, drainage density, stream length density. The tributary morphology is also behaving different characteristics from north to south along the middle Teesta river within the Lesser Himalaya. Tributary fan are present on both the side of Teesta river but from our mapping it is indicated that maximum number of tributary fans are belong to western bank. Also they have more prominent fanlobes up to three fanlobes surfaces. Fanlobes are mapped morphologically and stratigraphically as F0- F1-F2 which represent active fanlobe, fanlobe Level 1 and fanlobe level 2 respectively. Fanlobe F1 is younger than the Fanlobe F2. Fanlobe F2 shows more thickness of surface and older. Fanlobe F1 has less depostional thickness with different surface area distribution as compared to the other fanlobes level. Active fanlobes has variable size of its surface area in most of the tributary depends on its tributary basin characteristics. Each and every fanlobes shows toe-cut parallel to the Teesta river channel. Palaeo toe-cut demarcates the axial channel margin marked in the distal fanlobe of the older tributary fanlobe. There are three phases of toe-cut in three fanlobe surface of the most of the tributaries stream fan . Fan entrenchments are present along the stream fan channel and they incised with different rate in each fanlobes. From the three entrenchments level and toe cut it is understand the region has three phases of upliftment. In addition to this evidence the Teesta valley shows three main terrace level adjacent to the tributary fanlobe in the confluence. Terraces are generally present in between the tributaries confluences with main Teesta river. Terrace surface are describe as T1, T2 and T3 from younger to older surface. From the terrace level it is also suggested that these region has three phases of incision. Deposition sequence of tributary fan and the terrace is comprises of four main facies flows i.e. Hyperconcentrated flow, debris flows, Channel flows and sheet Flows. Episodically erosion and deposition occurred in the region as both the main river and tributary fan provides imprints of climate change and tectonic upliftment. A significant link with axial river and tributary fan can be observed in the distal fanlobes which shows floodplain materials of axial river overlapping the distal part fanlobe. Evidence has proved that the high water and sediment discharge in the axial river can eroded as well as deposited partially in the distal fanlobes. Such evidence indicate that the region is influenced by both climatic and tectonics in the Lesser Himalaya.