The river valley, rarely more than 50 miles wide, is studded with numerous low, isolated hills and ridges that abruptly rise from the plain. The vally is surrounded on all sides, except the west, by mountains and is intersected by many streams and rivulets that flow from the neighboring hills to empty into the Brahmaputra.
Earthquakes are a common phenomenon in Assam. In modern times, the most important Assamese earthquakes have been those that occurred in 1897 with the Shillong Plateau as the epicenter; in 1930 with Dhubri as the epicenter; and 1950, with Rima in Tibet at the Arunachal border as the epicenter. The 1950 earthquake is considered to have been one of the most disastrous earthquakes in history. It created heavy landslides that blocked the course of many hill streams.
The average temperature is moderate, about 84 degrees F (29 degress C). Assam does not have the normal Indian hot, dry season. Winter sets in from around the end of the month of October and lasts till the end of February. The temperature drops to a minimum of 6 to 8 o Celcius, the nights and early mornings are foggy, and rain is scanty. Summer arrives in the middle of May accompanied by high humidity and rainfall. The temperature reaches a maximum of 35 to 38 o Celcius. Thunderstorms known as Bordoichilla is a frequent occurrence during the afternoons. Spring and Autumn with moderate temperatures and modest rainfall are the best seasons. Some rain occurs from March onwards, but the real force of the monsoon winds is faced from June onward. Rainfall in Assam ranks among the highest in the world; its annual rainfall varies from 70 inches in the west to 120 inches per year in the east. Large concentrated during the months from June to September, it often results in widespread destructive floods.
Assam's forests cover about 20 percent of the total area. The Kaziranga National Park, the stronghold of the fast-disappearing great Indian one-horned rhinoceros, is its most famous wildlife refuge.