1 Introduction
The main threats to freshwater fishes are due to habitat alteration, pollution, overexploitations and introduction of non-native species causing their global decline (Nguyen and De Silva, 2006). The conservation of freshwater habitats and the associated fauna has increasingly become a priority (Allan and Flecker, 1993; Richter et al., 1997). It is imperative to inventories the freshwater fishes at both local and regional levels; so that, the conservation status of the species could be ascertained and the nature and magnitude of threats identified (Pethiyagoda, 1994; Dudgeon, 2000). This diversity is being eroded each day mainly because of unending anthropogenic stress. It is not only the wealth of India and the world but it also has serious implications on fishery domain.

Rivers are highly sensitive to any change, tectonic climatic conditions of an area. As such, they are the most powerful indicator of these activities. The tectonic deformation causes the changes in channel slope, which, in turn, is responsible for variation in channel morphology, fluvial process and hydrological characteristics of a river system (Jain and Sinha, 2003). River Siang is spreading into the different tectonic and litho-logic domain. The river system also provides a unique opportunity to study the evolution of fluvial landscape (Talukdar, 2011).

2 Study site
The River Siang, is largest river of Brahmaputra river system, originates from Chema Yungdung Glacier near Kubi at 5150 m in Tibet (Figure 1). In Tibet it is popularly known as Tsang-Po, flows in West–East direction. After traversing a distance of about 1625 km river in Tibet and then it takes a turn in south direction, enters the territory of India near Tuting in the Upper Siang district of Arunachal Pradesh and flows through North-South direction in East Siang district towards Assam and finally it merges with Lohit and Dibang in Assam and it becomes the mighty River Brahmaputra (Das et. al., 2014 a; Das and Kar 2015).

Figure 1 Map of (A) India indicating Arunachal Pradesh, (B) Arunachal Pradesh indicating to East Siang District, (C) In East Siang district highlighting River Siang (Study Area) of Arunachal Pradesh

3 Materials and Methods
Habitat inventory parameters were recorded in a standard format (NBFGR, 2000) from each study spot in the field itself. The geographical co-ordinates and altitude above mean sea level (m.s.l.) of study spots were recorded with the help of Garmin made GPS-60. Habitat features were quantified using transects within each stream reach. Variables measured at each point of transect were water depth and width of the river and current speed. In each study point of the stream, in a reach measuring 100 m in length (the longitudinal path following the fastest cross sectional water velocities) micro-habitats were identified, classified, (such as, Fall, Cascade, Riffles, Pools and Run, sheet), and various measurements taken (Armantrout, 1990; Kar 2007, 2013; Arunachalam,1999; Das and Kar, 2011).

The study area covers a major part of East Siang District of Arunachal Pradesh latitudes 28002/43.16// - 28⁰10^/28.87^//N and longitudes 95⁰13^/05.30^//- 95⁰21^/39.00^// E and altitude 501- 1681 Ft.; which is included in the
Survey of India (SoI) topographic map nos. 82 O/11, 82O/12, 82 O/15, 82 O/16, 82P/9, 82P/10, 82P/11, 82P/12, 82P/13, 82P/14, 82P/15, 82P/16, 83M/5 and 83M/9 on 1:50,000 scale. The satellite Images LISS III were collected from the NRSC Hyderabad, India, Bhuvan and also from USGC. The present study is confined to both the plain and hill areas because drainage anomalies in the plains and hills indicate the influence of subsurface structures, which are active in nature (Saha et al., 2006; Saders and Tabuchi, 2000; Das et al., 2014 b).

4 Results and Discussion
River Siang, a hill-Stream of 1^st order river; had colluvial (landslide from adjacent hill slopes delivering sediments and organic matter) valley segment and pool-riffle type of reach. This reach type was most commonly associated with small to mid-size streams and was a quite prevalent type of reach in the rheophilic zones. On the other hand, runs were generally laminar flow of water with sandy substratum. Pools, riffles and runs were generally found to dominate the micro-habitat type with frequent occurrence of trench pools. River Siang was said to be more entrenched based on V-shaped valley segment. The substrate type had been found to be dominated by boulders, gravels and cobbles with frequently-occurring quite large number of boulders and some bed rocks. This kind of habitat with concomitant physico-chemical characteristics of water was said to be a suitable feeding and breeding ground for Tor spp. and Neolissochilus spp. 

The substrate of River Siang was dominated by boulders. The level of embeddedness was high for large boulders and low for small cobbles. About 57% of the sample site was covered by boulders, 18% by thick layer of cobble, sand and gravel 24% each and remaining by silt and clay particles 1% showed in Figure 2. The boulders and cobbles provided hiding and spawning place for many hill stream fish species.

Figure 2 The Substrate Composition of River Siang

The riparian type was generally mesoriparian while the riparian vegetation type is generally deciduous trees with some amount of shrubs and grasses. Different types of fish covers had been recorded in River Siang ranging from undercut bedrock to bottom-free big boulders and pools. Riparian land use pattern mainly involves human habitation and some amount of agriculture through ‘Jhum’ cultivation. Field observations revealed differences in habitat preferences among the fishes of different size groups.  Nevertheless, the fishes in River Siang seem to prefer deep pools as one of their favourable habitats in view of depth, providing strong cover to them. However, they also make use of covers like bottom-free big boulders, undercut banks, etc. (Table 1). Riparian vegetation did not seem to provide much cover to these fishes.

Table 1 Habitat Characteristics of River Siang

Habitat Inventory of River Siang, in the province of Arunachal Pradesh, was studied in six sectors, two sectors each in the upstream, mid-stream and downstream stretches. The study revealed differences in valley segment among the different sectors of the River Siang. In the plain region of the river, the valley segment was alluvial; while, in the true hill stream stretches i.e. in the upstream regions of the river, the valley segment was colluvial. The latter makes the habitat colonisable for the rheophilic fishes, most of which had developed adhesive apparatus on the ventral surface of the body in order to withstand the fast torrent of the river. Further, colluvial valley segment provides a diversity of micro-habitats while the alluvial valley segment generally showed only regime type of reach. Moreover, the River Siang was generally wider where there was alluvial valley segment. The fish covers in the studied river had been found to be mostly turbulence and depth. In some regions, fish cover was dominated by overhanging vegetation. The substrate type in the hilly portion of the River Siang was found to be dominated by gravels, cobbles, boulders and bedrocks; while, in the plain portion of the river, the substrate type mostly consists of fines. It was mesoriparian in the River Siang.  But the riparian vegetations vary from shrubs to trees in the different regions of the river. Signs of erosion had been predominantly visible in all the studied segments of the River Siang.

In general, it had been found that, species diversity increases from upstream to downstream regions in the streams. According to McConnell (1988), this was due to the fact that, habitat diversity also increases along the stream gradient; but, may not be in the same direction and fashion, as species diversity. In addition, detritus and plankton richness also seem to increase downstream which may lead to greater diversity in the higher trophic levels. Certain studies on African stream fish communities (Toham and Teugels, 1997) revealed the occurrence of distinct pattern (s) in the longitudinal zonation of species with a sequential shift in species compositions and also an increase in species diversity along the stream gradient. Further, spatial segregation of fishes was said to be influenced by environmental and physical characteristics of the stream like width, depth, current velocity and substrate type (Toham and Tuegels, 1998); as well as, certain biotic characteristics like canopy cover and the occurrence of the in-stream detritus.

However, riffle and pool communities were linked by a continuous gradient; at a very local scale and species may use both these habitats at different parts of their life cycles, as well as, during different seasons (Schlosser, 1982). Nevertheless, some of the juvenile fish may prefer riffles to pool habitats; as they provide the required kind of food and flow patterns necessary at a particular stage of their growth; whereas, the same species may prefer pool habitats at later parts of their life cycles. Further, there had been some observations on size-related differences in habitat use by many species where smaller individuals of some species occur in faster-flowing waters than their larger conspecifics (Toham and Tuegels, 1997). Information about the habit and habitat of these fish species in the River Siang was scarce; and, were based mainly on the present study. Data about the reproductive period, sites of breeding, feeding, etc., was also scanty. The food of the adults generally consists of herbivorous items with occasional carnivorous components. The gut contents revealed the occurrence of mainly phytoplankton and zooplankton food (Table 1).

The regional drainage pattern of the area under study is nearly parallel to sub-parallel, whereas a prominent annular pattern is observed in the central part of the area (Figure 3). An attempt is made here to examine how are drainage networks influenced by underlying neotectonically active structures (Roy, 1975). In River Siang catchment area the major drainage can be delineated as dendritic, sub-dendritic, sub-parallel, trellis and rectangular pattern. The drainage of the most of the part of the river catchment area is collected from the available topographic maps. Drainage was controlled by the structure and the lithology of the area.

Figure 3 Drainage Basin Map of River Siang from the upstream to downstream of the river

The drainage networks by large rectangular and trellis that are orienting the channels along North-East and East-West directions in the central domain. The drainage patterns are as follows (Figure 4-6, Table 2):
Ⅰ. Dendritic Pattern
Ⅱ. Trellis Pattern
Ⅲ. Rectangular pattern
Ⅳ. Sub-Parallel Pattern

Table 2 Different types of Drainage Pattern of River Siang and their Geological Significance

Figure 4 Dendritic Pattern

Figure 5 Braided Pattern

Figure 6 Meandering Pattern

The course shifting and braided character of the alluvial river also gives some anastomosing pattern of channel. In the eastern side Ranaghat and Pasighat shows anastomosing pattern. In the eastern side of the river change its course occupying the other tributaries along with the main river (Talukdar, 2011; Ouchi, 1985). This type of anastomosing is found in intermittent to perennial stream systems with net long-term ero-sion, in contrast to braided streams which are characterized by net long-term deposition, and which occur within well-defined floodplains (Figure 7).

Figure 7 Drainage System in Different Year

Acknowledgement
The authors are thankful to Assam University, Silchar and to the UGC, New Delhi for granting UGC-Fellowship to the First author.

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