River flood plains are among the most fertile and richest ecosystems on the earth, thus, it has been very essential to manage properly. Biogeomorphological state of any river flood plain indicates the usefulness of it. Study of herbaceous vegetation (biomass, species types) through quadrat method, NDVI, is significantly helpful to bring out the present real scenario of biogeomorphological state of a river flood plain area. River flood plains are important landscapes features for retaining river nutrients and sediment loads [1] . The river flood plain is typically defined as narrow strip on each side of the river channel [2] . Thus, flood plain lies between natural levee and flood basin and is lower in elevation than that of natural levee [3] . This is primarily due to periodical floodplain inundation [4] , thus is seasonal recruitment habitat [5] which replenishes the soil nutrients and sustains a high level of landscape diversity. River flood plains are environments that alternate between wet and dry phases, with lower lying parts being wet than dry and higher parts being more dry than wet [6] . Flood plains are habitat for terrestrial biota during dry phase, and in wet phase, flood plains are habitat for aquatic fauna and mobile species such as fishes, grasses, etc. [7] . Historically, humans have used rivers flood plain more than any other types of ecosystem [8] . The term biogeomorphology was first used in the eighties [9] ; biogeomorphology is a relatively new discipline within the study of river flood plain system. Biogeomorphological (flora and fauna based on the development of land formation) state of any landscape helps us to understand the fertility, productivity as well as the total ecological scenario of the specific region [10] . Herbaceous vegetation species’ (density, biomass) study based on satellite imageries provides an idea of river flood plain ecology [11] . Biomass measurement of vegetation (herb, shrub and tree) using quadrat method is an important ecological property to understand the river flood plain landscape pattern [12] . Vegetation types related to basin geomorphology will provide a basis for future re- storation planning regarding river flood plain [13] .

The Teesta river flood plain presents a very typical biological and geomorphological characteristic and behavior. This river channel represents a normal flow of the moribund and mature hydrological situations of a flood plain environment. Although the Teesta presently carries a very poor discharge of water, still it is the major river of the northern part of Bangladesh influencing greatly in the agricultural sector and as a major tributary of Jamuna River [14] . The Teesta river flood plain plays a vital role in the overall ecological and socio-economic life of Gangachara upazila. At least 75,000 [15] thousands of people Gangachara upazila are somehow dependent on the Teesta and its biogeomorphological settings. It is a time of concerning to study about river flood plain for ensuring the current human demand due to rapid growth of population and industrialization. River flood plain of Gangachara upazila developed by Teesta river flowing from the northern part of Bangladesh toward southern part as major tributary of Jamuna river is biogeomorphologically as well as geopolitically significant to study. That is why assessment of herbaceous vegetation biomass using quadrat method and NDVI using Landsat satellite image were adopted to find out the present biogeomorphological state of Teesta river flood plain area.

River flood plain is one of the most significant landscapes that historically roles in livelihood of human being including socio economic as well as cultural aspects through its diverse and effective ecological settings that is why it is considered as biological super market. At present, river flood plain is more concerning issue throughout the world. In this research the study area biogeomorphologically is facing vulnerable situation day by day as it is losing adequate amount of water discharge from upper stream due to geopolitical issues between Bangladesh and India. It is time to think about the problem to manage the flood plain area. Teesta river flood plain is the most significant landscape in the north- western part of Bangladesh that falls in the old Himalayan piedmont plain comprising of recent flood plain deposits [16] . The Teesta river flood plain in the Gangachara upazila presents a very typical biological and geomorphological characteristics or behavior. The present study area map provided from BWDB is located in the Gangachara upazila under the Rangpur district of northern part of Bangladesh. Figure 1 shows that Gangachara upazila is located between 25˚48' and 25˚57' north latitudes and between 89˚05' and 89˚21' east longitudes covering an area 272.28 sq. km where total land area of 206.45 sq. km, is bounded by Kaliganj (Lalmonirhat) and Jaldhaka (Nilphamari) upazilas on the north, Rangpur Sadar and Kaunia upazilas on the south, Aditmari and Lalmonirhat sadar upazilas on the east, Kishoreganj (Nilphamari) and Taraganj upazila on the west.

The aim of the study was to assess the present biogeomorphological state of the Teesta river flood plain area. The present research work was based on the combine application of field survey, lands at satellite image processing, flood plain extension mapping, etc. Firstly, total river flood plain area has been estimated 68.5 sq. km, from that flood plain area, herbaceous vegetation samples have been collected to analyze biomass state where total sampling area is 104 sq. feet and total 31 types of herbaceous species are identified. NDVI operation has been run to find out the mostly vegetation covered area classified into dense vegetation and no vegetation covered area; the DNs value of NDVI is 0.475242 representing high vegetated areas and the DNs value −0.0774711 shows no vegetation covered area in the Gangachara upazila. However, based on the analyzing of herbaceous vegetation biomass, NDVI, the present biogeomorphological state of Teesta river flood plain area under Gangachara upazila was assessed, where especially the condition of herbaceous vegetation was assessed based on showing species types and biomass in dry and before dry condition. It has been found that, Teesta river flood plain changes its real nature due to many types of human interruptions and loses its biological status. So, it is a burning issue to study its current biological state. In accordance with that, the herbaceous vegetation study and NDVI operation have been adopted. This research will provide important guidelines

for better use of river flood plain area as well as will assist river planners, researchers, engineers, administrator, geographer, farmers, and local inhabitants in their respective fields fruitfully.

Authors would like to acknowledge SPARSSO, BWDB and chairman of the Department of Botany of Carmichael College for providing necessary data for the purpose of completing the research work.

Islam, S. and Sar- ker, S.C. (2017) Assessing Biogeomorpholo- gical State of the Teesta River Flood Plain: A Study on Gangachara Upazila, Rangpur, Bangladesh. International Journal of Geosci- ences, 8, 265-275. https://doi.org/10.4236/ijg.2017.82011