Chalus is the Caspian side of the mountain, and the change you feel on arrival is almost physical. After the dry heights of the Alborz, the air thickens and turns humid, the hillsides are covered in green forest, and rice paddies and tea plantations appear on the lowlands. The town has about seventy thousand people, with Nowshahr only a few kilometres to the east, and it sits on a narrow coastal plain between the mountains and the sea. It is a holiday town for Iranians, who flock here in the summer to swim and eat grilled fish.
The road down is the finale of the Chalus route. After the long climb from Tehran and the tunnel at the top, it descends through the beautiful Chalus valley, with the river running beside you in a gorge. The corners tighten, the forest closes in, and the temperature rises. Fog is common near the top and rain is frequent on the northern side, since the mountains catch the moisture from the sea. The surface is mostly good but can be slick, and heavy trucks and weekend traffic are a constant. Take care on the descent, and do not follow the locals in their overtaking.
The Caspian is the world's largest inland body of water, and it is not a normal sea: it is salty but much less than the ocean, and it is home to sturgeon, whose roe is the famous caviar. The coastal plain grows tea, rice and citrus, and the forests of the Hyrcanian region, which stretch along these slopes, are an ancient survivor of the broadleaf forests that covered the area millions of years ago. They are a UNESCO World Heritage listing for good reason.
For fuel and food, Chalus has everything: pumps, restaurants, hotels. Kebab is a staple, but here you should try the local specialities: fish, pomegranate-walnut stew called fesenjan, and rice dishes with herbs. Evening on the shore is relaxed, with families walking and eating ice cream. If you can, stay overnight and enjoy the green, wet, slow-moving world on the north side of the mountain.