September, at least the early part, is a good time to seed grass waterways in most years. Whether it is a good time this year may depend on when and how much it rains. You'll need rain to get the grass germinated, but not so much that it washes out the seed, or even the newly formed waterway.
Check with your Natural Resources Conservation Office to see what recommendations they can offer for either starting a new waterway this fall or reworking an existing waterway. Check with the local Farm Service Agency office, often located next door, to see if funding is available through the Continuous Conservation Reserve Program, to help cover the cost of installation. Through that program, you are paid a rental fee for the land taken out of production.
The only trick is you may be too late to apply for a waterway now and still get it installed and seeded this fall. You may need to check now for a waterway you want to install next spring or summer.
Several companies offer straw mats that you can put down and staple to the soil to help hold the seed and soil in place until the grass can take hold and grow. Your NRCS office or local soil and water conservation district office should have information about where you can find these mats. Some SWCDs even help make mats available. Others own machines that allow you to blow straw over a waterway after seeding, and rent the units to landowners.
If you're not ready to seed a waterway this fall, at least use this time to determine if a waterway would help solve soil erosion problems where water concentrates in your field, even if you no-till. Sometimes a grass waterway is the right answer. Sometimes a sediment and water control basin that sends water underneath the ground through a tile to an outlet is the preferred method. Your NRCS office can provide technical assistance to help you figure out what will work best.