Pete Piringer, county fire department spokesman, said the woman fell into the trench, which runs parallel to a sidewalk, Sunday evening. He said it was dark when she fell in, and there was no barrier between the sidewalk and ditch.
The trench is about 6 to 8 feet deep, and is a couple hundred feet long.
Piringer said the woman had been out with family members, walking their dog. As she fell, a couple feet of the dirt wall gave way, with some coming down on her.
Rescue personnel treated the woman for mild hypothermia and took her to a hospital with possible leg and spinal injuries.
Authorities said construction workers doing utility work recently dug the ditch.