EFFECTS (IMPACTS) OF TEHRI DAM

By S.C.Sharma, Former General Manager THDC Ltd.

1. INTRODUCTION

The probable effects of human activities regarding the use of environmental resources on natural environment is called environmental impact, and the assessment and evaluation of environmental effects of human activities are called environmental impact assessment (EIA). Thus, EIA is an instrument to identify and assess the potential environmental impacts of a proposed project, evaluate alternatives, and design appropriate mitigation, management and monitoring measures. EIA should also be conducted for social environments, i.e., impacts of project on human beings.

The main activities involved in EIA are :

i) Identification of likely impacts. ii) Conduction of baseline studies to collect various available information related to the particular impact. iii) Impact prediction and assessment. iv) Impact interpretation or evaluation. v) Identification of monitoring requirements and mitigating measures.

2. ENVIRONMENTAL IMPACTS OF TEHRI DAM

The Project has identified both positive and negative impacts. Efforts were made to get full advantage of positive impacts by formulating suitable schemes.

As we know that dam is essentially an artificial wall constructed across a river which converts a running water ecosystem into a lake type ecosystem. This causes some changes in basic riverine ecosystem. Therefore, thorough studies were got conducted for the likely negative impacts of dam and its reservoir, through expert agencies. Mitigating measures, where necessary, were taken on likely negative impacts.

Probable impacts identified for detailed studies were :

a. Likely change in the (i) water chemistry, especially with respect to dissolved oxygen and (ii) turbidity of