How to Measure a River Table for Epoxy

• Mark the river length first so your width points are evenly spaced.
• Record widths every 6 to 8 inches or more often on complex bends.
• Use the deepest realistic target pour depth for planning.

• Use quick mode when the river width is fairly consistent.
• Use segment mode when the channel opens and narrows noticeably.
• If you are spending heavily on resin, segment mode is usually worth the extra minute.

Split a 72 in channel into six 12 in sections. For each section, estimate its average clear width from more than one reading; a reading at a sharp bulge alone is not an average. Suppose the widths are 4, 6, 8, 7, 5 and 6 in, with a consistent 1.5 in depth. The widths sum to 36 in, so volume = 12 × 36 × 1.5 = 648 in³ = 2.81 US gal before allowances. This matches the river calculator's equal-segment model.

The calculator applies one depth to the whole river. Using the deepest point across a sloping bottom is a conservative estimate, not the exact volume. For better accuracy, divide the cavity into sections with their own measured depth and add length × width × depth for each. Do not multiply all widths by the full table length, and do not treat unequally spaced width readings as equal-length sections.

In the example, being wrong by 1 in about the average width changes raw volume by 72 × 1 × 1.5 / 231 = 0.47 gal. That is about a sixth of the estimated channel quantity. Measure inside the finished mold, subtract occupied wood and inserts, and check dimensions after final positioning. Keep a photograph and the measurement sheet so a later discrepancy can be traced to geometry, absorption or loss rather than guessed at.

Use L for the full channel length and w₁, w₂, w₃ for widths at representative equal-length sections. If the sections have different lengths, use Σ(length × width) divided by total length, or calculate each separately. Width sampling cannot reveal hidden voids or a sloping mold floor; those require their own measured allowance.