One of the questions with which I am repeatedly faced regards ephemeral streams, washes, and arroyos. These systems have an ordinary high water mark (a easily noted bed and bank), clearly convey flow, can be only a foot wide, but are dry most of the year, only run in response to a storm event, have no vegetation or upland vegetation only, and are written off by most consultants, regulators, and the general public as having limited to no value.
In the generally accepted classification schemes for wetlands and riparian habitats, we talk about functions and values. Functions are a fairly easily defined activity conducted by the aquatic resource, such as infiltration to the water table. Values are the socially-defined worth we give to that function or resource. My value can be different from your value, which is why we don't usually use values in the whole aquatic resource regulation scheme.
Clearly, headwater ephemerals do something. They convey water and sediment, they allow some infiltration. But what I find little of in the literature is any other function they conduct in the whole biogeochemical scheme of southwest aquatic systems (related to water quality and system function as opposed to strictly fluvial geomorphology). There is research that suggests (I don't have it at hand, otherwise I'd cite it here) that headwaters in wetter systems conduct some pollutant attenuation and act as a buffer between uplands and the perennial reaches, and there's plenty of research on biogeochemical activity in an intermittent-to-perennial stream system in AZ (Sycamore Creek) but I haven't seen any research in the southwest on strictly ephemeral systems, other than grey literature on what flood event to which the OHWM actually corresponds (Lichvar, et al, Corps of Engineers ERDC-CRREL). (Funding is probably an issue; Sycamore Creek is lovely and inviting, an ephemeral wash is not so much, what hypothesis to test, durability of equipment in the summer monsoons, etc.)
With such limited information, it is hard for consultants/regulators to have concern regarding the regulation of ephemeral systems, much less argue with clients/applicants that piping, wholesale filling, or otherwise constricting the inconveniently-located dry washes in their development is not a good idea. We've been trained to think "green is good, brown is not" in our aquatic values (inherent in Clean Water Act regulation - wetland is better and more precious).
I'd love to have a biogeochemical argument other than flooding/water conveyance as to why it is important to avoid/preserve our ephemeral headwaters here in the southwest. There's nothing I like better than to walk through a large wash in the lower Coloradan, Sonoran, Mojave, and Colorado plateau deserts on a spring day, with wildflowers abounding, birds chirping, and lizards scrambling away from the day's sunshine.