🔍 Key Concepts

• How river gages relate their readings to a reference plane (like Low Water Reference Plane) • How to interpret a negative gage reading (is the water above or below the reference?) • Simple addition/subtraction between the gage reading and the reference plane height

💭 Think About

• If the reference plane is at 3.0 feet and the gage reads -0.5 feet, is the actual water surface higher or lower than that plane? • Think of 0.0 on the gage as being at the reference plane: where would -0.5 feet be on that same scale? • Draw a simple vertical number line with 0.0 at the reference plane, then mark -0.5 and see if that point is above or below the plane.

✅ Before You Answer

• Be clear whether a negative reading means ‘below zero’ on the gage or ‘below the reference plane’ • Confirm in your sketch which direction is up (positive) and which is down (negative) • Double-check whether the question is asking ‘above/below the plane’ rather than the total elevation above some chart datum