For kayakers, a strong roll is highly recommended. Group assistance for rescue is often essential but requires practiced skills. Risk of injury to swimmers is moderate to high, and water conditions may make self-rescue difficult. Scouting may be necessary the first time down. Rapids may require 'must make' moves above dangerous hazards. A fast, reliable eddy turn may be needed to initiate maneuvers, scout rapids, or rest.

Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure.

Intense, powerful but predictable rapids requiring precise boat handling in turbulent water. Rapids that are at the lower or upper end of this difficulty range are designated Class III- or Class III+ respectively. Injuries while swimming are rare self-rescue is usually easy but group assistance may be required to avoid long swims. Scouting is advisable for inexperienced parties. Strong eddies and powerful current effects can be found, particularly on large-volume rivers. Complex maneuvers in fast current and good boat control in tight passages or around ledges are often required large waves or strainers may be present but are easily avoided. Rapids with moderate, irregular waves which may be difficult to avoid and which can swamp an open canoe. Rapids that are at the upper end of this difficulty range are designated Class II+. Swimmers are seldom injured and group assistance, while helpful, is seldom needed.

Occasional maneuvering may be required, but rocks and medium-sized waves are easily avoided by trained paddlers. Straightforward rapids with wide, clear channels which are evident without scouting. Risk to swimmers is slight self-rescue is easy. Few obstructions, all obvious and easily missed with little training. Details of portages may be given if these pose specific challenges.Ī summary of river classifications as presented by the American Whitewater Association: Class I:įast moving water with riffles and small waves. While a river section may be given an overall grading, it may contain sections above that grade, often noted as features, or conversely, it may contain sections of lower graded water as well. Often a river or rapid will be given a numerical grade, and then a plus (+) or minus (-) to indicate if it is in the higher or lower end of the difficulty level. The grade of a river may (and usually does) change with the level of flow. For instance, there can be difficult grade twos, easy grade threes, and so on. The scale is not linear, nor is it fixed. (“Parts per million” refers to the number of carbon dioxide molecules per million molecules of dry air.) These measurements are from the mid-troposphere, the layer of Earth's atmosphere that is 8 to 12 kilometers (about 5 to 7 miles) above the ground.There are six categories, each referred to as grade or class followed by a number. Note how the map changes colors as the amount of CO 2 rises from 365 parts per million (ppm) in 2002 to over 400 ppm currently. The animated map shows how global carbon dioxide has changed over time. This is greater than what naturally happened at the end of the last ice age 20,000 years ago. Since the beginning of industrial times (in the 18th century), human activities have raised atmospheric CO 2 by 50% – meaning the amount of CO 2 is now 150% of its value in 1750. The second graph shows CO 2 levels during Earth’s last three glacial cycles, as captured by air bubbles trapped in ice sheets and glaciers. The first graph shows atmospheric CO 2 levels measured at Mauna Loa Observatory, Hawaii, in recent years, with natural, seasonal changes removed. Human activities have raised the atmosphere’s carbon dioxide content by 50% in less than 200 years.Ĭarbon dioxide (CO 2) is an important heat-trapping gas, or greenhouse gas, that comes from the extraction and burning of fossil fuels (such as coal, oil, and natural gas), from wildfires, and from natural processes like volcanic eruptions. Carbon dioxide in the atmosphere warms the planet, causing climate change.

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