The Undershot Water Wheel Design, also called a "stream wheel" was the most popularly utilized kind of waterwheel designed by the old Greeks and Romans as it is the least difficult, least expensive and simplest kind of wheel to build.

In this kind of waterwheel design, the wheel is set directly into a fast-flowing river and supported from above. The motion of the water below makes a pushing activity against the submerged paddles on the lower part of the wheel enabling it to rotate one direction just relative to the direction of the flow of the water.

This type of waterwheel design is commonly utilized in flat places with no natural slope of the land or where the flow of water is sufficiently fast-moving. Compared and the other waterwheel designs, this kind of design is inefficient, with as little as 20% of the water's potential energy being utilized to rotate the wheel. Additionally, the energy of the water is utilized just once to rotate the wheel, after which it streams away with the rest of the water.

Another benefit of the undershot water wheel is that it requires enormous amounts of water moving at speed. In this way, undershot waterwheels are normally put on the banks of waterways as smaller streams or brooks do not have enough potential energy in the moving water.

Another benefit of the undershot water wheel is that it requires enormous amounts of water moving at speed. In this way, undershot waterwheels are normally put on the banks of waterways as smaller streams or brooks do not have enough potential energy in the moving water.

One method for improving the efficiency slightly of an undershot waterwheel is to divert a percentage off the water in the stream along a narrow channel or conduit with the goal that 100% of the diverted water is utilized to rotate the wheel. So as to accomplish this the undershot wheel must be thin and fit precisely inside the channel to keep the water from getting away around the sides or by increasing the size of the paddles.