Summary – Ch 9: River Network Generation

20+Times Asked

~160Total Marks

14+Numericals

CriticalPriority

Exam strategy: This is the highest-scoring numerical in the course. The same pattern appears in almost every paper (10–12 marks). Given a DEM grid: find sinks, fill them, derive flow direction, compute flow accumulation, extract river network with threshold, and calculate watershed area.

1 Complete Watershed Delineation Workflow

Fill Sinks: Identify cells lower than ALL 8 neighbours. Raise to min neighbour + 0.5 m.
Flow Direction (D8): Each cell points to steepest downhill neighbour. Codes: E=1, SE=2, S=4, SW=8, W=16, NW=32, N=64, NE=128. Diagonal: divide by √2.
Flow Accumulation: Count upstream cells flowing into each cell. 0 = ridge.
Stream Definition: Accumulation × cell area ≥ threshold = river cell.
Watershed Delineation: Trace divide boundaries.

2 Sink Identification and Filling

A sink is a cell lower than all 8 surrounding cells.
Must be filled to ensure continuous flow paths.
Fill to minimum neighbour elevation + 0.5 m.

3 Flow Direction (D8 Method)

Eight possible directions with power-of-2 codes.
For diagonal: slope = (elev diff) / √2.
Edge cells: only consider existing neighbours.

Direction codes: E=1, SE=2, S=4, SW=8, W=16, NW=32, N=64, NE=128. Draw arrows from each cell pointing to its flow direction.

4 Flow Accumulation

Start from ridge cells (accum = 0), trace downstream, add counts. High accumulation = stream channels.

5 Stream Threshold

Threshold: Accumulation × Cell_Area ≥ Min_Catchment_Area
Example: 100 m × 100 m = 10,000 m². Threshold 40,000 m² → need ≥4 cells accumulation.

6 Watershed Area

Count cells in each watershed × cell area = watershed area. Largest watershed = most cells draining to outlet.

7 Stream Ordering (Shreve)

Exterior (source) links = order 1.
At confluence, orders are added (unlike Strahler).
Order 1 + Order 1 = Order 2; Order 2 + Order 1 = Order 3.

Q Past Exam Questions

+Given a DEM grid: find sink, fill, flow direction, flow accumulation, river network, watershed area. (2071–2082, 8–12 marks)

(a) Check each cell against 8 neighbours. Fill sink to min_neighbour + 0.5.

(b) Calculate slope to each neighbour. Arrow to steepest descent. Show all arrows.

(c) Start from 0-accum cells. Trace downstream, count upstream contributors.

(d) Apply threshold. Cells ≥ threshold = river.

(e) Trace divide lines. Count cells per watershed. Area = count × cell_size².

+Explain the flowchart of watershed delineation. (2076 Bhadra)

DEM → Fill Sinks → Flow Direction → Flow Accumulation → Stream Definition → Stream to Feature → Pour Point → Watershed.

+Explain Sink, Flow Direction, Flow Accumulation, River Grid, Stream Lines. (2074 Magh)

Define each briefly. Sink: depression. Flow Dir: D8. Flow Accum: upstream count. River Grid: binary raster. Stream Lines: vector conversion.

+Explain the Shreve method of stream ordering. (2075 Bhadra)

Exterior links = 1. At confluence, orders summed. Gives magnitude of drainage network.

+Steps to generate river network from multiple DEMs. (2082 Shrawan)

Mosaic → clip with buffer → fill sinks → flow direction → flow accumulation → threshold → convert to vector → clip to boundary.

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