Field survey of a beel: transect data and an embankment inlet
Class XII students survey a beel (wetland) near Nagaon. Along a north–south transect they record: north shore, elevation 62.4 m, water depth 0.8 m, paddy; middle-north, 61.8 m, 1.6 m, fishing; centre, 61.2 m, 2.3 m, open water; middle-south, 62.1 m, 0.5 m, reed bed. Between July and November the water level falls 0.7 m. The students also count 35 households around the beel; 60% depend on fishing and 25% on paddy. A road embankment across the western inlet has no culvert. Villagers report that fish catch has declined since the embankment was built. The school has a survey compass, measuring tape, ranging rods and a hand-held GPS, but no echo sounder.
1. Describe the cross-section shown by the data and identify the landform. Which point is deepest, and what does the reed bed suggest?
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The transect shows a shallow basin: the margins are higher (62.4 m on the north shore and 62.1 m on the south) and the centre is lower (61.2 m) with the greatest water depth (2.3 m). This is consistent with a beel formed from an abandoned river channel or oxbow, where the centre preserves the old channel thalweg and the margins have been filled by silt. The middle-north fishing point is intermediate in both elevation and depth. The reed bed on the shallow south margin suggests silting and gradual encroachment of vegetation, which is a natural stage of infilling but can be accelerated by reduced inflow. The 0.7 m fall between July and November shows that the beel is strongly seasonal.
2. Assess the likely impact of the road embankment on the beel and on fish catch.
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The embankment without a culvert cuts the western inlet, so it reduces the monsoon inflow that fills the beel and connects it to the river. The beel will fill less, dry earlier and hold less dry-season water. Reduced flow also lowers the sediment-flushing capacity, so silt and reed encroachment are likely to increase. Fish catch can decline for two reasons: adult fish cannot migrate in from the river to spawn, and young fish lose the flooded margins that serve as nurseries and refuges. The villagers’ report is therefore consistent with a hydrological cause, not simply overfishing. Because 60% of households depend on fishing, the embankment is also a livelihood issue.
3. Design one legend symbol for the survey map and one follow-up measurement the students can make with their available equipment.
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Legend example: open water as a blue area; reed bed as a green stippled symbol; paddy as a yellow area with a rice-stalk symbol; fishing point as a small fish symbol; the road embankment as a black double line; the inlet as an arrow showing flow direction. Follow-up measurement: measure the inlet flow with a float and the measuring tape, using velocity × cross-sectional area at a fixed section before and after any culvert is built. Alternatively, repeat the transect with ranging rods and GPS to map shoreline retreat and reed expansion. The school has no echo sounder, so depths should be taken with a graduated pole at fixed GPS points rather than guessed.