Groundwater and springs on valley slopes: if the reduced level (R.L.) of the groundwater table on both valley sides is 1505 m and the stream R.L. at the valley floor is 1475 m, and a 60° slope consists of pervious soil only between R.L. 1485 m and 1500 m, at which R.L. is a gravity spring most likely to emerge?

Introduction / Context:
Gravity springs typically occur where a permeable stratum carrying groundwater outcrops on a slope and intersects an impermeable layer or the ground surface. Identifying the expected emergence elevation relies on the geology and the vertical placement of pervious and impervious strata relative to the groundwater table.

Given Data / Assumptions:

• Valley-side groundwater table R.L. = 1505 m.
• Stream bed R.L. = 1475 m.
• Pervious soil exists only between R.L. 1485 m and 1500 m on a 60° slope; layers outside this band are effectively impervious.
• Gravity flow discharges where the pervious layer daylight intersects the slope at its lower boundary.

Concept / Approach:
A perched/permeable lens drains along its downslope limit. The spring emerges near the lowest elevation at which the pervious layer is present and intersects the slope face, provided this level is above the local base level and hydraulically connected.

Step-by-Step Solution:
Identify the lower boundary of the pervious band: R.L. 1485 m. Recognize that pervious material above an impervious base will exfiltrate at the lowest outcrop. Therefore, the gravity spring is expected at or near R.L. 1485 m on the slope.

Verification / Alternative check:
The stream R.L. (1475 m) is below the pervious band; drainage from 1485 m will flow downslope to the channel, consistent with observed hillside springs at the pervious–impervious contact.

Why Other Options Are Wrong:

• 1500 m and 1505 m: Upper levels of the pervious/mounded groundwater—springs usually emerge at the lower outcrop.
• 1475 m: Stream level is below the pervious band; spring emergence occurs where the pervious intersects the ground, not necessarily at the thalweg elevation.

Common Pitfalls:

• Assuming springs always appear at the stream level rather than at the pervious boundary outcrop.

Final Answer:
1485 m.