Ho trovato una spiegazione in inglese dalla quale si evince che le alette non c'entrano niente.

Wing Deflectors
Wing deflectors are triangular shaped wedges of rock built to jut out from the bank into the channel. They point downstream at an angle to direct currents away from eroding banks toward midstream and to increase current speed. Water moving downstream slides off the deflector at an increased velocity. This fast-flowing current cuts and cleans a deep, narrow, center channel that increases sportfish habitat and protection from predators.

Deflectors are built singly to protect an eroding bank, or in pairs to narrow the stream channel. They are placed in a series, positioned alternately, on one bank and then the other to keep the current sweeping swiftly in a natural winding pattern. In nature, pools and riffles normally are repeated every 5 to 7 channel widths; therefore, the distance between deflectors should be at least seven times the average stream width.

Deflectors may be constructed entirely of rock, or rock and wire combinations (gabions), or rock framed with logs. Build deflectors in a triangular shape, with the tip pointing downstream and angling out toward the channel. Adjust the angle (usually about 30 degrees between the bank and the deflector) to guide the current into mid-channel at a suitable speed.