Drawing a river in Photoshop is very simple. The most important part is to draw organic lines instead of linear lines (straight lines) and to make it taper at one end. If you just draw linear lines, it would look like a road instead of a river. Tapering it adds depth and makes the river appear as if it is disappearing over the horizon. Once you have your river drawn, you can add other Photoshop effects.
Open Photoshop and click "File," then "New." Name the file "river" and set the dimensions you want for the river. Make sure the "Mode" is set to "RGB Color" and set the "Contents" to either "White" or "Transparent." Then click the "OK" button.
Check to be sure your layer isn't locked. If you can't see the layers panel, click on "Windows" in the main menu, then click "Layers." If the layer is locked, right click on the layer and click "Duplicate Layer," then delete the locked layer by right clicking and then clicking "Delete Layer."
Click on the "Pencil" tool in the tool bar. If you can't see it, it might be hiding under the brush tool. Click on the "Brush" tool and then the "Pencil" tool from the pullout menu.
Start at the bottom of your page, left click on your mouse and while holding your mouse down, draw an organic line to the top of the page.
Draw another organic line parallel to the first line, tapering it so the width is smaller at the top of the page and wider at the bottom.
Check to be sure your lines extend past the page. If you need to enlarge, hold down "Ctrl" + "T" on your keyboard to bring up the transform tool. Click on one of the square handles, and while holding down your mouse, drag to enlarge.
Create trees, rocks and other outdoor images to finish your scene.
Add style to lines before colouring by clicking on "Filter" in the main menu and then click on "Sketch." You can play with the different choices until you find something you like or hit cancel to go back to your original. Add colour to your river by clicking on the colour palette and choosing a colour of your choice, then click on the "Paint Bucket" in the tool bar and left click between the two lines to fill the river with colour. Add ripples and other special effects by going to "Filter" on the main menu and clicking on "Distort." Then click on any effect from the pullout menu to bring up the editing screen. From the editing window you can add effects such add ripples, twirls and more.
If you have problems colouring your river, pay closer attention to step 6. Colour will spill over when you have open lines.