One of these parks is Powell Butte.
From the park's pamphlet:
Powell Butte's recent history begins with the purchase of the 556 acre Wilson Homestead in 1925 by the City of Portland. The City's early recognition that Powell Butte's location and elevation would be of high value as a water reservoir site was first utilized in the 1960s when two above ground tanks were constructed by Powell Valley Road Water District. This was followed by Portland Water Bureau's completion of a 50 million gallon underground reservoir in 1980, and a 66 inch water transmission line linking Powell Butte to Washington County in 1983.The park is now about 600 acres, and includes large meadows and dark stands of Douglas Fir and Western Red Cedar, all through which wind miles of trails for hikers, bikers and horse back riders.
The reservoir is barely noticeable.
There are parts of the park that are off limits to humans, for the sake of wildlife. They do get out and about though, so it's possible to see deer, several species of birds, bugs, and reptiles.
You get the full range of meadow and forest plant life too.
If you live in Portland, or are in the area, spend a couple of hours wandering around on the Butte. It's one of the gems of the Portland Park system.