River Rock Calculator

River rock is naturally rounded alluvial gravel and cobble, sorted and abraded smooth by stream transport, then screened into nominal size bands such as 1 to 2 in, 2 to 4 in, 4 to 8 in, or 6 to 12 in cobble. Estimating it is a volume problem: plan area in square feet times depth converted from inches to feet gives cubic feet, dividing by 27 gives cubic yards, and multiplying by a bulk density of roughly 1.40 to 1.55 tons per cubic yard gives tons. That bulk figure describes the placed bed, not the mineral: the parent rock is near 2.65 specific gravity, but a loose landscape bed is 35 to 45 percent air, and larger stones pack at slightly different void fractions, which is why density creeps upward across the size range. Depth drives both cost and performance. Two inches is about the minimum that shades soil enough to suppress weed germination; 3 in suits 2 to 4 in stone in beds and borders; and 4 to 6 in is used for dry creek beds and swales built from 4 to 8 in stone, where a working rule is a depth of at least twice the largest stone dimension. At 3 in deep, a ton of medium river rock covers roughly 70 to 80 sq ft, dropping to 50 to 60 sq ft at 4 in. Caveats: rounded stone migrates on slopes steeper than about 3:1 and needs edging or angular material instead, bagged 50 lb quantities cost far more per ton than bulk delivery, and published coverage figures vary 10 to 15 percent with gradation and placement.

The stone itself is a product of fluvial transport: clasts are abraded and rounded during downstream travel, which is why roundness and sphericity became formal sedimentology measures in the twentieth century, notably in the classification work of William C. Krumbein in the 1940s. Size terminology still follows the grain-size scale published by Chester Wentworth in 1922, under which material from 4 to 64 mm is pebble and 64 to 256 mm is cobble. Commercial river rock is dredged or pit-mined from these alluvial deposits and screened into those bands rather than crushed, which is the reason it keeps its rounded faces. Decorative stone as a landscape surfacing material spread widely in the United States after the Second World War, pushed by suburban development, the ready availability of screened aggregate from highway-era quarry expansion, and the shift toward low-water landscaping in arid regions that accelerated after the droughts of the 1970s. Woven and non-woven geotextiles, developed for road and drainage work in the 1950s and 1960s, were adapted as landscape fabric and became the standard separation layer beneath stone beds. Imported polished material such as Mexican beach pebble entered the market later as a premium graded product, which is why its price per ton commonly runs two to three times that of locally sourced river rock.