Boat Wake Erosion Is Expensive. It washes away thousands of dollars worth of land. It buries and kills fish eggs, plants, and invertebrates and disturbs waterfowl nests. Dredging downstream docks & channels also costs money. If you stand on shore when a speedboat goes by, you can see sediment lifted away by each wake.
Wakes erode 8 Square Feet per Gallon of Gas, or 50 Square Feet per Hour
Erosion per Hour. Each speedboat trip peels a few thousandths of an inch off shorelines. This is a lot of land, because boats travel and erode for several miles each hour. Some boats go straight; some go back and forth, eroding the same spot. Either way, boating for an hour at 20 mph erodes 20 miles times 3 thousandths of an inch times two sides of a river = 50 square feet.
These figures are illustrative. The table at left shows that boats often erode more than .003 inches off the shore. They erode less only if they go very slowly. Wakes have been peeling land away in places like the Chesapeake Bay, Intracoastal Waterway and Norfolk Broads for decades, and you can read the studies. Erosion per Gallon of Fuel. Most boats travel 2 - 4 miles per gallon (see list), compared to cars which travel 20-40 miles per gallon. Boats' huge fuel use goes into pushing water down and sideways, causing wakes. The wakes release their energy on shorelines, causing erosion. When a boat travels 3 miles on a gallon of gas, it peels off the shore 3 thousandths of an inch times 3 miles times two sides of a river = 8 square feet per gallon.
The cost of boating includes losing your own and others' land. Losing 8 square feet per gallon, for land worth $2 per square foot, is a land cost of $16/gallon. (Land values of $2/square foot = $90,000/acre. Some riverfront areas are worth much more, some less.)
Some boats go farther on a gallon, like the 16-foot Maryland boat in the table, which goes over 6 miles per gallon, so it erodes 17 square feet/gallon = for a land cost of $34/gallon.
Canoes and sailboats erode much less, because they make tiny wakes.
How to Protect Shores. Boats can go dead slow (erosion starts about 2 mph) or travel in a stone-lined area. Canals and harbors (Washington, Venice, Shanghai, etc.) are stone-lined to reduce erosion from constant boat traffic. Erosive energy does not decrease much with distance from shore. Landowners need plants on the shore wherever possible, not rock or walls, to keep water from speeding up & damaging downstream banks, and to protect habitat along the shore. If rocks are placed, dirt can be packed among them and protected with anti-erosion cloth while plants grow in the dirt and over the rocks.
Storms & Floods. Storms & floods do wash some land away. On the other hand floods rise over riverbanks, and drop the soil they carry on top.. Floods and storms have been happening for millennia, so river widths have reached a balance between the soil taken and delivered by storms.
Unlike floods, boat wakes do not raise the water level, so the sediment they erode stays on the river bottom and is not placed on top of riverbanks. Wakes make the water shallower and wider as they erode land away.
Natural processes move rivers without widening them, by depositing soil on gentle natural slopes inside each curve while the river cuts on the outside of curves. In contrast, boat wakes erode the inside of curves, because they focus wake energy on the inside when a boat rounds the curve. A steep slope inside a curve, or a river getting wider, are symptoms of boat wake erosion.
Dangers from Boat Propellers & Noise. Propellers can cut swimmers, fish, mammals and vegetation. Propeller guards provide protection. Propellers and engines also create harmful underwater and surface noise.