Thanks for the question! There are some very easy-to-follow routes for waterfall lovers. I recommend you either order a free copy of the Oregon Scenic Byways magazine or view it online as it really outlines the routes with beautiful pictures. As far as the time of year, there will be more water at other times of the year (winter, spring) however in August there is still more than plenty of water for the waterfalls.
From North to South
I-84 and the Columbia River Gorge: This is the iconic waterfall route in Oregon that includes Multnomah Falls (the states tallest). The waterfalls in this area are breathtaking and also easy to access from the road. They tend to be more crowded given the proximity to Portland.
Silver Falls: Given the short distance of the Columbia River Gorge, I suggest visiting the falls and then backtracking. If you love farm land, make your way south through Mollala and Silverton to Silver Falls State Park. This might be one of the best places to visit for waterfalls and hiking. It’s just beautiful here.
Highway 126 – Florence to Sisters: Sweet Creek Falls is one of my favorite waterfall hikes is near Florence and the Oregon Coast. The numerous falls parallel the trail and in the summer months the water is warm enough to wade into. If you continue inland along Hwy 126 you pass Eugene and head into the McKenzie River Valley. About an hour and a half drive from Eugene, you will arrive at Sahalie and Koosah Falls, two beautiful falls that are along the McKenzie River National Recreation Trail. You can park at either waterfall and hike in a loop to see both.
Bend: Highway 126 east links up with Highway 20 which heads into Central Oregon and leads you towards Bend. There are some beautiful falls in Bend, including Tumalo Falls. From here you can head south along Highway 97 to Highway 138 west. Highway 138 offers access to the north entrance of Crater Lake (this entrance is only open in the summer months) and parallels the wild and scenic North Umpqua River. One of the highlights of the waterfalls on this route is Toketee Falls. There isn’t much of a hike to get to this waterfall, but there are other waterfalls along Highway 138 with some more hiking. If you want to hike here, I recommend seeing the falls and then cross the road to hike along the North Umpqua Trail (just watch out for poison oak, which is extremely thick in this area).
I hope this helps you plan a great trip. Let me know if I can offer any more suggestions.