As a river transitions to less steep land, it gets wider and slower and... - receives more rain and pollution, and fewer photosynthetic organisms
- contains less dissolved oxygen and more photosynthetic organisms
- gets cooler, more sunlight, and more photosynthetic organisms
- contains more oxygen, more nutrients, and fewer algae blooms

Stimulus is something that causes a reaction, especially interest, excitement or energy. An example of stimulus is a shiny object for a baby. Receptors in muscles provide the brain with information about body position and movement. The brain controls the contraction of skeletal muscle. The nervous system regulates the speed at which food moves through the digestive tract.