Sandy River Delta Park, better known by locals as Thousand Acres, is the off-leash mecca of Portland. With 1,400 acres of open field, wetlands, blackberry bushes, countless trails, and two rivers where your dog can play fetch, splash around, and run free, this is a dog's paradise.
That being said, the Sandy River Delta isn't exclusively a dog park. In fact, this piece of property sandwiched between the Sandy River, Columbia River, and Interstate 84 is open to mountain biking and horseback riding. The area is also popular with birders (there is a bird blind) and waterfowl hunters, and it is even a great place to lay out a towel for a swim in the Columbia. Further, about a third of the park has been set aside to protect wildlife habitat and is completely off limits to any human or canine activity.
A part of the Columbia River Gorge National Scenic Area now under management by the U.S. Forest Service, the park was once the site of an aluminum factory, and it was more recently used for grazing cattle. Once under jurisdiction of the Forest Service, the main task at hand has been to restore the property to its native estuary wetland ecosystem, which means removing invasive species such as Himalayan blackberry and reed canary grass, while at the same time maintaining the off-leash regulations.
Note: Dogs are required to be leashed in the parking lot and on and within 100 feet of the Confluence Trail. The park is open from dawn until dusk.