River Road East is a dirt/gravel road that runs from the main park road near the Rio Grande Village to the Mariscal Mine. Despite the name, you won't be able to see the Rio Grande from most of the trail, but you'll have views of the Sierra Maderas del Carmen mountains off in Mexico to the east and the Mariscal Mountains to the west.
Vegetation on River Road East is sparser than some of the other primitive roads in the park, and there is little protection from wind on a windy day. Like most of the other dirt roads in the park, you wouldn't ride this trail for its technical merit or fun factor, but this is a far better way to see the park than from the window of your car.
The route shown starts at the Mariscal Mine which is the highlight of the route. It's definitely worth exploring, but keep in mind this was a cinnabar mine. Cinnabar is a toxic ore of HgS (mercury sulfide), and mercury is also extremely toxic, so take appropriate care.
As you travel towards the Rio Grande, the landscape changes from desert scrubland to grassland near the river. Once River Road East turns north (after the intersection with Solis Road), the surface becomes covered in white crystals that really reflect the sunlight. Then as you continue north towards the main park road, it switches back to the desert scrubland more typical of the park.
River Road East is typically open to 4×4 vehicles, but high clearance may also be required - check with the NPS for current conditions. From the Mariscal Mine, you can continue on Talley Road
to either Black Gap Road
or River Road West which eventually (27.5 mi) takes you all the way to the Castolon Visitor Center.
Keep in mind this is a desert, and it can be very hot most months of the year. There is no potable water available along the route, so you'll want to carry more than you think you might need and there is no shade available. Winter is a great time for this ride, but the desert can get surprisingly cold in the winter, and the sun can still be intense especially in the section covered in white crystals. If you're unsure of conditions, check with a ranger.