If the scale of the map is such that this is noticeable, you might consider drawing the rivers as polygons, rather than lines. Use the Change like draw tool in the right click menu of the Change Properties button and pick a river. Then hide all the other sheets and use the TRACED keyboard command to trace a river system. You will have to delete the original river line, but now you have a polygon instead of a line you can change the mouth of the river to be as wide as you like.

If you mean trouble trying to click exactly on the coastline, you can use the F9 key to attach the end of the river to any point on the coast as you draw it.

If you mean the visual mismatch that sometimes occurs where there are different sheet effects on different sheets, you might consider adding a Color Key sheet effect to the Land sheet, converting all the rivers to solid magenta, then moving them onto the LAND sheet, where they would then cut a line through the land. This is only realy successful if you haven't covered the land all over in terrain textures, or haven't done it yet so that you can avoid going over the rivers when you do.

reversing the relative position of the land and sea sheets and drawing all the water on top of the land is Another way of doing it, but this is quite involved and means changing the background to land texture, redrawing the ocean on a new sheet above the land and terrain sheets, and then drawing the rivers on the same sheet as the ocean. There are advantages and disadvantages to doing it that way, but some of the more recent overland styles in the Cartographer's Annual have sea over land like that. You would need to put the land sheet effects on the ocean sheet instead and invert their direction - inside where it was outside on the land, and vice-versa. That's what I mean about being rather involved.

I have seen maps in the past where mappers have carefully drawn little river mouths over the join between river and sea ona new sheet and tried to blend the two things together that way, but again that is quite time consuming to do and doesn't always work terribly well.

@JulianDracos - you can draw your own houses in any bitmap editor and then create map files for them. The information on how to do this is contained in the Tome of Ultimate Mapping, and you can always ask for help here. I've done hundreds myself.

Thanks for your help, Joe :)

I have managed to reduce the sideways drift by removing one of the Displace effects, the Blur and Blend mode. I think that for now this is the best I can do, and suspect that I was pushing the effects to do unexpected things by pushing them a long way beyond what is normally expected of them. This is a much better result, and I will probably stick with this one now.

I boosted the height factor of the normal map when I made it, because I wasn't getting enough swirliness. Maybe that is affecting the result?