Fluvial Geomorphology — Bed Scour Features When the zone of maximum stream velocity is close to the bed, material is excavated there. The resulting feature is best termed:

Introduction / Context:
Rivers sculpt their beds and banks according to local velocity and turbulence. Alternating zones of erosion and deposition create a characteristic sequence of pools and riffles in many alluvial channels. Understanding where scouring occurs helps interpret habitat diversity and channel stability.

Given Data / Assumptions:

• Maximum velocity near the bed increases shear stress and erosive power.
• Bed material can be entrained, deepening local depressions.
• The question seeks the correct geomorphic term for such excavations.

Concept / Approach:
Where flow concentrates and near-bed velocity peaks, shear stress exceeds critical thresholds for grain motion, causing scour. The eroded depressions are commonly called scoured pools (or simply “pools”). In contrast, riffles are shallow, coarse, higher-friction segments that often occur between pools; ox-bow lakes are abandoned meander loops formed by cutoffs; a floodplain is a broad depositional surface adjacent to the channel, not a local bedform.

Step-by-Step Solution:

Verification / Alternative check:
Hydraulic geometry and sediment-transport texts illustrate pool–riffle spacing approximately 5–7 channel widths, with pools forming at bends or constrictions where scour is focused.

Why Other Options Are Wrong:

• riffles: Typically depositional, shallow, coarse features with lower local scour depth.
• ox-bow lakes: Not a bedform, but a cutoff feature away from the active thalweg.
• flood plain: A lateral depositional surface; unrelated to near-bed scour pits.

Common Pitfalls:
Equating “fast water” with riffles only. While riffles have high surface velocity, the question emphasizes excavation by near-bed maximum velocity, which typifies scoured pools.

Final Answer:
scoured pools