I'm very impressed that you've been able to dig so much before the rainy season starts! Doesn't the clay soil bake solid? Our ground -- quite sandy in most of the areas we've been digging, but lots of clay pockets around -- gets nearly impossible to dig when it hasn't rained in months here, so digging our water movement earthworks and garden swales and
berms for our latest garden area was a months-long process as we were able to knock it out without killing ourselves.
What kind of trees are your tall thorny trees, did you say? Our only woody mulch source is also thorny: our mesquite trees, also affectionately known as "murder bushes." Since it's what we have, we just use it and try to avoid the thorns as much as possible. We wear our many scratches with pride. ;)
We also base our earthworks on trial and error and repeated observation. We've found that the best (and often the only) time to dig and modify our canals and swales is when it's raining and actively flooding and filling these channels, so we expect to get wet and muddy!
For what it's worth (and this may well not be kosher
permaculture, I don't know), we allow "weeds" -- grasses and Palmer's amaranth, mostly -- to colonize the banks of our water redirection canals to prevent erosion but don't actively plant anything in them. The water moves along them at quite a clip when it rains a good amount and keeps the bottoms of the ditches bare of plant material. This helps keep our driveway and footpaths from flooding and leading that floodwater directly to our front door.
We slow the water down first using grade and branching (i.e. many different options of places for the water to go) when it reaches our garden areas, then the garden swales are heavily mulched and planted mostly with nitrogen-fixing and fast-growing beans. We primarily plant tepary beans because they love our desert-with-monsoon climate, and we
seed them into the walls of the swales (our swales are quite deep and narrow, between 18" and 2 ft. deep by about 1 ft. wide on average, and deeply mulched, so we don't seed the bottoms). Cowpeas/black-eyed peas also do pretty well in our swales, and we've got a common pole bean called mechudo that's a pinto-black cross that requires a longer season but does pretty well if we add mulch-buried greywater drip irrigation lines to the monsoon floodwater (this is true of our older garden area). We plant yucca stalk teepees in the berms near the mechudos so they can climb and shade other parts of the garden. We plant mixed squash near the tops of the swales as they meet the berms -- the "shoulders" -- as well as some devil's claw (
Proboscidea parviflora) to hold soil, provide shade, and
bear fruit.
Our swales surround "tree islands" of pre-existing (but increasingly happy) mesquite, where we've also planted other native and non-native food-bearing trees and bushes in sunken beds and will be planting more prickly pear in raised sand and gravel beds.
In the last couple of heavy rains, we observed that excess water is still flooding slowly out of the top/back of both garden swale areas, so I just started digging what I'm calling a "tuber delta" at the upper end of the feeder canal to the newest garden area after it climbs a little and becomes the last mulched and planted garden swale. It widens out into a deep delta from the extended swale, and I'm surrounding it with a berm. Once I've finished digging it out, I'll add
compost, then chop and drop, then woody mesquite chip mulch, and I'll plant all the roots and tubers in there that I haven't been able to plant elsewhere because digging them up would mess with the careful grading of all the other planting areas. I don't have pictures of this yet, and the digging is slow-going even though the ground is softer now in rainy season because I can't handle that intense sun for too long, so I dig in little bursts as I can.
Apologies that some pictures below are blurry -- the low light was a challenge for the fancy camera.