Can't Seem To Wrap My Head Around River Placement

[saltqueer]

I'm starting my first ever worldbuilding world with this small hot spot archipelago. It's at about 43 deg S on a world that is, for my convenience, basically the same as Earth. It mostly has a Csb climate, with Dsb above 2500m, and polar above 3500m, on the tops of the 3 main volcanoes, and a rain shadow (Bsk, I believe) on the east side of the biggest island. I'm having LOTS of problems figuring out where to put rivers. Nothing I do looks right. I read through the How to get your rivers in the right place thread about 5 times before determining I had better just ask y'all directly. I'm also not really sure if I have enough granularity in my elevations, but I'm imagining them as mostly smooth between the topo lines for now. That might also be part of the issue? My main issue seems to be figuring out where watersheds would be, and then the rivers mostly? seem to follow from there?
Here's the map so far, if a higher quality image is needed I can get one, I just didn't want to go unnecessarily big
islands-big.png

If it matters, the three tallest volcanoes are active, with mostly fairly long spaces between eruptions, and the little one to the east that barely breaks 1250m is the youngest of the bunch, and pretty active as well. The geology is mostly based on Hawai'i, and the climate is based on southern Oregon, as that's the closest real-world analogues I could find for those.

[Redrobes]

Welcome to the guild.

The rivers tutorial:
https://www.cartographersguild.com/s...ead.php?t=3822

Look at your hills and work out the direct route down the sides. Where water collects is where your rivers and streams will be. If they all collect up in a bunch you have a lake and it will exit out to the sea eventually. Rivers dont generally split so where it could pick the biggest and make that dominant and dry up the other exits.

[waldronate]

Because the islands are small, the rivers are small (the longest river in Hawaii is 28 miles long, according to its Wikipedia page). Because these islands are tall, rainfall will vary wildly from one place to the another, both because of wind and because of altitude. Climate conditions (including prevailing wind direction and available moisture) may vary over the lifetime of the island, so there might well be old channels for rivers that aren't there anymore. Certainly, some lava flows will adjust river drainages on the younger islands. The highest parts of the volcano are unlikely to have lots of annual rainfall simply because they are above most of the clouds.
The second attachment shows the relationship between trade winds and rivers in Hawaii. The first attachment shows some potential river flows based on a rough model of altitude based on your map (it most emphatically doesn't provide any model of wind or rain or temperature effects: it treats the entire area as getting a uniform amount of moisture per point on the map). Once you get an idea of the prevailing wind directions and rainfall amounts, you can possibly use some of the information on the first attachment to determine where rivers would be (prune away the parts that would be dry).

[KMAlexander]

I can't remember where I found this, but it's a handy guide for how river systems tend to form (this is based on Californian systems.)

04GFsAjLVS14fYcwtPFVqwmbomidAEaGLZCOAa5_gvs.jpg

Note the footnotes - Parallel, Annular, and Rectangular are uncommon—at least in California. Distributary systems form when a river reaches sea level and breaks apart. Because of modern engineering and consisting dredging to keep shipping lanes open, this fracture doesn't happen as often as it used too.

[waldronate]

Those drainage patterns are just the general patterns that you're likely to encounter anywhere, not just in California. Dendritic is flow on relatively uniform flat areas, parallel is flow on relatively uniform sloping areas, trellis is flow on undulating terrain (usually old ones that cross over the ranges of hills), rectangular is incised jointing in large blocks, annular is flow around a hill in the middle of a parallel pattern, and radial is flow down from a hill onto a relatively flat area. A distributary is something else entirely and only happens at the outlet of fairly large rivers (they are comparatively tiny when held against most of the other flow patterns and certainly tiny compared to the river's overall length). Distributaries have the delightful property that they're constantly changing as the individual channels silt up and move unless herculean efforts are expended to force the pattern to remain the same. Those efforts invariably cause all manner of problems in the long term (see the Mississippi delta).
For a volcanic island, radial is the most likely pattern of flow that you'll encounter because it's all downhill from the top. A compound volcano like the big island of Hawaii is likely to have some small areas with parallel drainage because the cones can coalesce into a relatively flat slope (see the rivers on the big island's north side).

[KMAlexander]

Oh, totally. But the page comes from a book on Californian systems, which is why it leaves out a few. (Centripetal, Deranged, and Angular.)

100% agree Radial is the way to go for volcanic islands.