In the wet season it seems like drainage would be really important so they don't flood and overflow.
In the dry season it would be nice to take the best advantage of the few rain events that occur.
This makes me wonder if there wouldn't be some way to get better drainage in the winter and better water retention in the summer.
Perhaps in the fall, pull away much of the soil at the bottom of the swale -- say, up the sides some.
Then in the spring, replenish the trough with soil and heavy compost then mulch.
Plant annuals and enjoy.
This would preclude perennial plantings where this technique is employed, so maybe only do this in the middle (lengthwise) section and use perennial plantings at the edges.
Maybe a mixture of evergreen shrubs which can use the winter water and deciduous trees for the summer.
But maybe I'm thinking too much of the water as a problem.
Maybe the key is to find the right plantings for the climate of the Willamette Valley.
Wetter areas supported Oregon ash, Douglas-fir, bigleaf maple, black cottonwood, and an understory of poison-oak, hazel, and Indian plum -- maybe we'll skip the poison-oak.
Another question, what would happen is part of the swale valley were significantly deeper than the rest? I guess the problem is that if the slope is more than 1% or so along the bottom you might get too much erosion and it would just fill up. So unless it was a very long run it would not allow much depth variation. Hmm.