Our final WWotW, as part of our erosion identification series, is “Gully“.  A gully is a feature characterized by the erosion of land by water cutting sharp edges into soil, typically on a hillside.

The photo above illustrates the severity in which gullies erode the landscape.  As mentioned in our WWotW on rills, gullies are basically the same feature, but much larger. You can put your fingers in a rill, you can step into a gully.

Gullies usually form on steeper slopes and/or with a large volumes of water in a short period of time.  Summer thunderstorms are the perfect scenario for gullies to form.  The rapid removal of sediment from the landscape and potential deposition into waterbodies is a major environmental concern as the eroded material can carry nutrients and bacteria.


Sheet Erosion

Continuing our WWotW erosion identification, we take a look at “Sheet Erosion“.  Sheet erosion is the detachment of particles, usually by raindrop impact, and their removal downslope by water flowing overland in a sheet as opposed to defined channels like a rill.

Sheet erosion usually begins on less severe slopes as water flows over the land evenly carrying sediment particles downslope.  As the flow and slope increase, the sheet will consolidate and begin forming rills and gullies.

Sheet erosion can be easily mediated through the planting of vegetation to hold soils in place and slow the flow of runoff.


As spring moves forward steadily and summer approaches, I think it is important to dedicate the next few “WWotW” to erosion features.  This week we will look at rills.  Rills often form as the initial sign of erosion.  By definition, a rill is a narrow and shallow incision into topsoil layers, resulting from erosion by overland flow.

Rills often form on steep slopes with little to no vegetation.  The overland flow of water is generally not severe on steep slopes as rills are fairly shallow features.  A rule of thumb is that if you can only fit your hand in it, it is a rill.  If you can step in it, it is a gully.

The image above illustrates how sheet erosion (discussed next week) concentrates and forms rills which eventually grow to form gullies.

Oxbow Lake

An Oxbow Lake encompasses the best of both worlds; rivers and lakes!  Oxbow lakes are U-shaped bodies of water that are formed when a meander of a river is cut off and isolates the meander as a lake.

Young rivers generally start off fairly straight.  As time goes on, they erode their stream banks forming meanders in the river.  The older they get the more pronounced this meander becomes.  In some circumstances, this meander becomes so severe that the bank is eroded away permanently and cuts the meander off, once again forming a straight path for the river.  When this happens, an Oxbow Lake is formed.

A river with an extreme meander that will soon erode its bank and form an oxbow lake.

An aerial view of an Oxbow Lake next to the river it was cut from.

Alluvial Fan

In order to properly define an “alluvial fan“, we must first define “alluvium“.  Alluvium refers to clay, silt, sand, and/or gravel particles that are deposited by running water.  Debris from flooding is also considered alluvium after it has been deposited.  Therefore, an alluvial fan is a fan-shaped accumulation of alluvium deposited at the mouth of a ravine or at the juncture of a tributary stream with the main stream.

Alluvial fans generally form where a stream’s gradient decreases, when a fast moving tributary enters a larger, slower moving river, or where a stream flows into a lake.  When this happens, the stream deposits course grained material that cannot be suspended by the flow.  The deposition of course grained material then builds up and forces the stream to gradually build out in a conical or fan-shaped way.  As this happens sand particles begin to fall out of suspension as they cannot be carried by the flow and the process continues until eventually the stream deposits its fine grained material farther out.  The result is an alluvial fan.

A large scale alluvial fan in the Tibetan Plateau.

A small scale alluvial fan formed at the edge of a river.


A “snowpack” is formed as layers of snow accumulate in regions of high altitude or during the course of the winter season.  Upper Colorado River Basin snowpack.  Snowpacks can be used to date events and evaluate climate.

Around the world, snowpacks are an important water resource that feed streams and rivers as they melt.  During normal years, they help recharge surface and groundwater for the upcoming summer.  In years where minimal snowfall accumulates, these water resources can be depleted early or not exist in the first place.

Assimilative Capacity

Sometimes referred to more simply as “assimilation“, assimilative capacity is the capacity of a natural water body to receive waste water, toxins, or polluted runoff without harmful effects and damage to aquatic life and humans who consume or use its water.

Lake Winnipesaukee Assimilative Capacity Analysis for Lake Waukewan and three main Winnipesaukee bays.

The chart above shows the current phosphorus concentrations in blue, the amount of phosphorus each water body can take on before being impaired in white, and a 10% buffer on those numbers as lakes and bays don;t necessarily go impaired right at 7-8 parts per billion phosphorus.  This information can be found in the Winnipesaukee Gateway.

When water bodies have a remaining assimilative capacity, they are usually considered to be high quality waters as they can handle more nutrients and pollutants before a water quality decline is observed.


Invasive Species

In honor of it being National Invasive Species Awareness Week, our Watershed Word of the Week is Invasive Species.  An invasive species is any plant or animal species that was introduced to an area that it is not native to.  These species tend to disrupt and adversely affect the habitats and bioregions they invade economically, environmentally, and/or ecologically.

In our area, one of the most common and recognized invaders is Eurasian Water-Milfoil.  Milfoil is thought to have been introduced in the 1940’s and is now present in every state in the United States.

Invasive species are extremely good at taking over an area for several reasons.  1. They reproduce quickly. 2. They have no natural predators. 3. They grow rapidly and displace native species.  With just these three advantages, it is no wonder when an invasive species is introduced, they take over and are extremely difficult to remove.

Invasive Species in Maine (Maine Invasive Species Network, University of Maine):


Common Name Species Name Source
European Fire Ant Myrmica rubra Univ of Maine / Maine NAP
Gypsy Moth Lymantria dispar GMRI – Vital Signs
Spotted Wing Drosophilia Drosophilia suzukii Univ of Maine
Forest Pests including Emerald Ash Borer, Asian Longhorn Beetle, Hemlock Wooly Adelgid, Elongate Hemlock Scale, Brown Spruce Longhorn Beetle, Winter Moth, European Wood Wasp Agrilus planipennis, Anoplophora glabripennis, Adelges tsugae, Fiorinia externa, Tetropium fuscum, Operophtera brumata and Sirex noctilio Maine Forest Service

The Maine Forest Service wants to know about any of the species of forest pests listed above. Contact them at (207) 287-2431 or 1-800-367-0223 (in State).

Not sure what you have? Visit UMaine Cooperative Extension – Homeowner IPM Pest ID Page or the Insect and Plant Disease Diagnostic Lab.

Marine Invaders

Common Name Species Name Source
Asian Shore Crab Hemigrapsus sanguineus Maine Dept of Marine Res
Dead Man’s Fingers Codium fragile GMRI – Vital Signs
Lacy Crust Bryozoan Membranipora membranacea GMRI – Vital Signs
Green Crab Carcinus maenas GMRI – Vital Signs
Hairy-clawed shore crab Hemigrapsus penicillatus GMRI – Vital Signs
Chinese Mitten Crab Eriocheir sinensis GMRI – Vital Signs
Common Periwinkle Littorina littorea GMRI – Vital Signs
Rapa whelk Rapana venosa GMRI – Vital Signs

If you see Asian Shore Crab, contact Peter Thayer (Maine Dept of Marine Resources) via email at  pete.thayer@maine.govwith the date, location, and number of crabs you have seen

Fish Introductions

Common Name Species Name Source
Northern Pike Esox lucius Maine Dept of Inland Fisheries

The illegal introduction of any fish into any Maine water is a Class E crime, punishable by fines. If you know of an illegal introduction, protect Maine’s fisheries and call Operation Game Thief at 1-800-253-7887 or visit the MDIFW website for more information. A list of “unrestricted” fish and wildlife species are listed by the Dept of Inland Fisheries and Wildlife for possession and sale.

  •  If you know of a good resource not currently included here please contact Caleb Slemmons ( to add a link.

Terrestrial and Aquatic Plants Identification Page.

Invasive Species of New Hampshire

If you think you have identified an invasive species please call the proper authorities to manage and deal with the issue.


Wetlands come in all sorts of shapes and sizes.  “Wetland” is a broad term that indicates an area with hydric soils (wet long enough to develop an oxygen depleted layer) and the ability to sustain aquatic plants.

A classic Maine wetland, a slow flowing river, hydric soils, aquaitc plants, and surrounded by forests.

Wetlands located on main lands (ie not near the ocean) can be classified as swamps, marshes, bogs, and fens based on the hydrology and vegetation.  A brief classification of each type of wetland is as follows:

Swamp – a wetland that is forested
Marsh – a wetland that is dominated by herbaceous rather than woody plant species
Bog – a wetland that accumulates peat, a deposit of dead plant material
Fen – a wetland that is fed by mineral-rich surface or groundwater

While there are several different types of wetlands, they are all extremely important ecological systems.  Primarily, wetlands serve to filter water, removing excess nutrients and sediments.  They also help buffer regions during flooding and to stabilize shorelines.

The image above illustrates the important functions of wetlands, including the dissipation of incoming stream energy (important during high flow times), breaking down contaminants, and filtering sediments and excess nutrients.

Wetlands are also considered to be one of the most biologically diverse ecosystems.  Many species of plants and animals live in wetlands.

Impervious Surfaces

An impervious surface is a surface that does not allow water to pass through it.  Most impervious surfaces are artificial or “man-made”, but some surfaces can become impervious naturally, like a herd path or a walking path from soil compaction.

Rooftops, parking lots, concrete, asphalt, and the list goes on and on, are all impervious surfaces.  The city pictured above (courtesy wikipedia) is a huge area where water cannot infiltrate back into the ground.  Where lots of impervious surfaces exist, polluted runoff tends to be found in much higher volumes.  When it rains, the water runs off the roofs and parking lots picking up gases, oils, trash, etc. and deposits it into storm drains.  These storm drains then carry the water to the nearest surface water, usually a river or the ocean.

The above photo ( shows an outfall of a storm drain loaded with trash and algae from excess nutrients.  

Impervious surfaces go against the natural process of things.  Rain water in pristine areas, with no urban development, soaks back into the ground where trees and grasses absorb some of it and the rest helps recharge our groundwater system.  When that water cannot infiltrate back into the ground, it can cause some major issues in the form of polluted runoff and erosion.

Whenever building new infrastructure, always consider where the rain water is going to go.  Will it runoff across the land into a body of water carrying harmful pollutants or will it  seep into the ground and be filtered by the sediments before entering our groundwater system.  Check out AWWA’s Youth Conservation Corps page for more information on water friendly conservation techniques.  Below is a picture of infiltration steps, one of the many techniques you can use to stop erosion and prevent polluted runoff from entering waterbodies.