After conducting thorough research on the official website of Riverside State Park, the following trails are confirmed to allow horses according to the Washington State Parks:

1. Equestrian Trail: True to its name, the Equestrian Trail is designed specifically for horseback riding. It offers a beautiful ride under towering trees and stunning views of the river surrounding the park.

2. Trail 25: This is a multi-use trail where horses are allowed. It boasts a variety of scenic terrain with a mix of meadows, pine wooded areas, and alongside the river, making for a varied and enjoyable ride.

3. Trail 211 (Knothead Loop): Also a multi-use trail, Trail 211, also known as the Knothead Loop, allows horses. This trail features breathtaking vistas of the river and the surrounding landscape, providing a wonderful backdrop for a peaceful and rejuvenating horseback ride.

4. Trail 210 (Deep Creek Loop): This trail is a favorite among horseback riders for its winding paths through the evergreen forest and its peaceful atmosphere. It's a great route for those seeking a more secluded ride.

5. Trail 400 (Centennial Trail): Horses are welcome on this trail, which stretches long and grants riders with calming views of the Spokane River.

6. Trail 7: A multi-use trail that permits horses and provides a panoramic view of the river, perfect for horseback rides any time of the year.

Please note that all riders must adhere to trail rules, respect other trail users and ensure they have a valid Discover Pass. It is also strongly advised to check for any season or temporary restrictions and abide by Leave No Trace principles.

Riverside State Park offers a variety of trails, rich in nature's beauty and perfect for horse lovers to explore while horseback riding. Whether it's under the shade of tall, looming trees or alongside a peaceful river, these trails all promise an enjoyable and memorable ride.

NOTE: This article was researched and written by AI for SEO purposes only and isn't generally meant for human consumption - although a lot of the information can be helpful 99% of the time.  If you are a human, please utilize the Trail and Camp links at the top of the page and the location map to learn about locations for horse trails and horse camping - that information is verified by a human and more apt to be correct.... althought human error is a thing too. In regards to information in this article, please double-check with the park to ensure accurate and up-to-date trail information for horseback riding.