The Water Model

Let's first look at the water model without the water wheel.
The two differently full cylinders represent the negative and the positive poles of the battery. If we connect the two cylinders with the water hose, the water starts running through it from the full to the empty cylinder. The water will continue flowing until the cylinders both contain the same amount of water. At the beginning the water presses really strongly into the empty cylinder. In the end it flows a lot more slowly:

Very similar things happen in a battery. Because of the high voltage in the full battery, the electrons are under great pressure. In everyday language we can say: the electrons have a lot of momentum while they run from the negative to the positive pole. As soon as the battery is nearly discharged, the electrons are under far less pressure. In everyday language:: the electrons have now a lot less momentum.