rivers and tides

For more Andy Goldsworthy-- In Missoula, there are lots of books at the University of Montana Library. Other interesting artists of this genre are
Hamish Fulton, Chris Drury, David Nash, Richard Long (see Land Art in the U.K.)

My personal favorite--Wolfgang Laib (Germany)--does installations with elemental materials like hazelnut pollen, beeswax, milk, rice, stone

Also Nils Udo (Germany)

Nancy Holt, Maya Lin (United States)

Cornelia Konrads

It's hard to not love Goldsworthy's stuff. For me, James Turrell falls in this realm as well.

http://www.pbs.org/art21/watch-now/segment-james-turrell-in-spirituality

Patrick Dougherty

Here's a great collection of collaborations with Nature.

http://pinterest.com/marytelera/land-art/

Showing kids Goldsworthy and related work is often interesting. Then set them loose in the woods, a meadow, or on a beach to create, and share the enthusiasm. It is great practice for observing/communicating with nature. It's also a wonderful exercise in leaving things behind as a gift to the world. I live in a densely populated area with lots of woods/parks and sometimes people correspond anonymously via nature art installations. And of course squirrels/birds/deer/etc are often apt to participate...