When a scene is reflected in water, it appears almost like an inverted mirror image.
Almost. But the reflection is different in a few important ways. First off, the light tones that you see in the scene above the water will appear a little darker in the reflection. These light tones might be clouds in the sky, a white house, or light-colored leaves on riverside plants.
The reason these light tones appear a little darker in the reflection is that some of the light penetrates into the water, rather than bouncing off the surface. This light is the very same light that you would see if you were snorkeling under the surface. If water were a perfect mirror, fish would live in pitch darkness! Because each parcel of light is reduced by the amount of light that is diverted into the water, the amount of light reflected is also reduced.
Note how the colors of both the blue sky and the orange bush darken when they're reflected in this wintry stream.
Water approaches the reflectivity of a perfect mirror only when you’re looking straight across it at a very shallow angle. As the steepness of the angle of reflection increases, the percentage of light entering the water also increases. If you are looking steeply down onto the surface of the water, not much light from the sky will be reflected. Think how dark the water in a lake or ocean appears when you look straight down into it from the side of a boat.
This light-eating phenomenon (called refraction, as opposed to reflection) came into play in this painting of a white resort perched above a lake. I was looking downward on the water, and was surprised how poorly the water reflected the white rocks along the lake's edge and the light stones on the building. I painted it the way I saw it, but it still looks strange to me.
As a reality check, here's a photo of the same place, shot with a steep downward angle. It shows the same effect, with most of the light tones disappearing into the water, rather than reflecting off its surface.