Riparian herbaceous vegetation is highly sensitive to environmental disturbances and serves as an effective indicator of ecosystem health and stability. The research was conceived from field observations along the Tsurang River under Changki village, where one riparian stretch is exposed to grazing, periodic flooding, and proximity to drainages originating from coal-mining areas, while another retains relatively natural forest vegetation. These contrasting conditions provided the basis for proposing a comparative assessment of herbaceous biomass as an indicator of riparian condition. Accordingly, this study assessed above-ground (AG) and below-ground (BG) herbaceous biomass at two riparian habitats along Tsurang River, Nagaland, North-East India: An Impacted Site (IS) exposed to anthropogenic disturbances and a Reference Site (RS) under relatively natural forest conditions. Herbaceous biomass was quantified monthly over one annual growing cycle (January 2023–January 2024) using the harvest method. The RS produced greater AG and BG biomass consistently, indicating more favourable soil moisture and nutrient conditions that supported greater plant productivity and root development. The IS, in contrast, was dominated by disturbance-tolerant grasses and opportunistic species that exhibited comparatively higher AG biomass during parts of the growing season with lower BG biomass. One-way ANOVA showed no significant seasonal variation in AG biomass at the IS (F = 1.329, p = 0.325) and the RS (F = 1.468, p = 0.288). However, BG biomass varied significantly among seasons at both the IS (F = 4.627, p = 0.032) and the RS (F = 4.004, p = 0.043). Mann Whitney test also found significant difference between the two sites BG biomass, p = 0.026 where RS generated a mean rank of 16.85, higher than IS (mean rank = 10.15) indicating greater BG biomass at RS. Both sites exhibited higher BG biomass during winter, suggesting increased allocation of resources to underground organs before the subsequent growing season. In providing an estimation of biomass repeatedly across one annual growing cycle, this study offers a detailed understanding of herbaceous productivity under contrasting riparian conditions. The findings demonstrate the potential of herbaceous biomass as indicators of riparian disturbance and provide baseline information for ecological monitoring, restoration, and conservation of riparian ecosystems in North-East India.