River Health Assessment of Mohana Watershed, Nepal: Implication for River Conservation

Abstract

Lowland rivers are being deteriorated from urban runoff, agricultural runoff, and industrial wastewater. Additionally in stream human activities impact water quality as well as ecological status of surface water bodies. This study examines seasonal changes on assemblage of macroinvertebrate communities and establishes relationship with the environmental variables and determines the river health of lowland rivers. Altogether six sites were sampled in tributaries of Mohana river for pre and post-monsoon seasons in the year 2017 and 2018. Water quality parameters were measured in in-situ by using the digital multi-parameter probe. The benthic macroinvertebrate samples were collected using multi-habitat sampling approach and identified upto family level. Redundancy analysis was used to explore environmental variables in relation to the macroinvertebrate communities. High density of macroinvertebrates was found in post-monsoon than pre-monsoon season i.e. 875 ind/m2 and 561 ind/m2, respectively while the diversity was recorded higher in pre-monsoon. The Ephemeroptera, Trichoptera and Odonata taxa were more abundant group in the rivers. Diptera was dominant taxa i.e. 43% and 66% in pre-monsoon and post-monsoon seasons, respectively. Moreover, there was no significant difference of macroinvertebrate abundance in both seasons but diversity was significantly different. Environmental variables like pH, EC, DO, TDS and water temperature were significant parameters influencing macroinvertebrates composition in low land’s river. The ecological status of lowland river of Mohana watershed was ranging from good to bad water quality class (i.e. ranging from 3.833 to 5.714 index value). The waste dumping, washing, and bathing, intensive use of chemical fertilizer and hydro-morphological alteration across the river section were the major stressor to deteriorate river water quality. Therefore, the water manager and local bodies should implement river management actions on time for the preservation of good status rivers for long-term use by the local people.