Seems to be a few comments from people who didn't watch TFV.<p>Yes, the idea is that you build a concrete wall that stays there forever. You don't just build one per stream - but place them every so often. Behind the dam the sand builds up after a few rains / years. Most silt, they say, flows over the top, so your downstream nutrient refresh still happens. Before you build the dam you lay down a network of pipes with small holes or slits that then acts as a horizontal spear / well with filtering of larger (sand) particles. In subsequent seasons the sand retains more rainwater for longer after each rain event, and that water can be recovered via that pipe network. Damming also has the benefit of slowing rain water down -- which reduces erosion and improves infiltration down and sideways from the reservoir.<p>The idea isn't new - low-head weirs have been popular (though not as popular as I'd like) for a very long time. Placement distances are determined by bed width, height, flow rate, seasonality, available funds, geography, etc -- so it could be every few hundred metres. Obviously concrete and earth moving aren't cheap, but waterways that only flow a month out of the year are much easier (and much more useful) to do this work on.<p>People worried about the fish - for non-permanent waterways it's moot. For low-head weirs on waterways that tend to always have flow, there's ways to allow fish movement in both direction.<p>Maintenance of the pipework could be an issue, but the nice thing about this approach is you put it in place, and it backfills and gets support during the next rain(s). I've looked at doing this on a low-flow / periodic creek bed before, and it'd involve some major earthworks to excavate down to below the normal water table (a couple of metres) and then the laying down of some very carefully constructed PVC piping - the big risk there is movement and cracking of same as you back-fill.<p>PVC pipes are typically used as they're easier to work with, and reasonably safe in that use case. Very small slits - 1 or 2mm - are made along as much of the pipes as possible, that allow water infiltration but should prevent sand. You need to pump-flush for a while to remove the proximal silt, but after that you've got an effective horizontal spear / well that is kind of self-filtering.<p>In this case there's nothing stopping you building another sand dam, a few hundred metres away, a decade later and repeating the whole process.<p>EDIT: Oh, for non-rendered images of the benefits of even just slowing seasonal water down in arid lands, search for "Geoff Lawton greening the desert" -- he's a permaculturist that's done some spectacular work in Jordan, with proven sustained / sustainable results.