Sediment Article by Elliot Lindsay

The nit and grit of sediment and streams

Sediment (soils, rocks, and other particles moving from the land into streams) is the focus of much discussion in the freshwater conservation world. How much is too much? When is sediment entering into streams a bad thing, or a good thing? There are a few important aspects of the sediment/water relationship that can help us better understand what is healthy or not, including:

Timing: when sediments are entering the stream, and how frequently they are entering the stream can shed light on whether it is a “natural” occurrence or not. Typically, sediment delivery is highest during the times of year when stream flows are highest. In SW Alberta, where this article was written, that’s usually spring and early summer. Our native fish evolved to deal with this by timing their spawning in the fall (like our chars – Bull Trout, Lake Trout) or in the spring (Westslope Cutthroat Trout) or summer (most minnow species) following high water, when the rate and volume of sediments ending up in the stream is low, or declining. Salmonids are particularly susceptible to sedimentation as their eggs are buried or settle into stream substrates (gravel etc.) and are prone to being “smothered” if there is too many fine sediments settling over these areas. Sedimentation outside the typical natural timing could be more impactful than during the usual high flow season for this reason.

The other aspect of timing is duration, “natural” sediment releases can be abrupt and can be quite extreme (e.g. landslides, bank failure, mass wasting, etc.), but they are also short term events that happen when streams are most able to manage this volume of material as flows are high and the sheer volume of water is able to transport this material onto floodplains or to downstream reaches where it becomes essential to the formation and maintenance of habitats. The other side of the sediment coin is that many watercourses, especially those which are heavily managed by dams, are actually sediment starved, cutting downward into the valley floor, experiencing significant erosion and disconnection with their floodplains which causes all sorts of ecological problems and loss of fish habitat. This is a common problem in urbanized watersheds, which can have a reduced supply of coarse sediments (gravels, cobbles) as streams are prevented from shifting sideways about their floodplain and eroding their banks (a natural process), and an increased supply of fine and very fine sediments (sand, silt) from roads, gravel lanes, construction sites, etc.

Quantity: the volume of sediment entering the stream is an important factor in how the stream deals with this material and whether it will have negative impacts. So many factors are interrelated – a little bit of sediment in the wrong place at the wrong time can be more impactful for some species than a large amount of sediment during the right time.

Type: sediment refers to a wide variety of materials from boulders all the way down to silts. When we see the brown rushing water in springtime, it’s usually because it’s full of fine sediments which are small particles able to be suspended in the current. Only during the highest flows can the largest streambed materials be moved; these are often referred to as channel-forming flows because the channel can shift and change as banks erode, while new banks are formed by deposited sediments.

Sediments are simultaneously responsible for spawning habitat creation (deposition of gravels and cobbles) and loss (deposition of fine sediments). Chronic delivery of fine sediments from human disturbances and landscapes is usually what we are concerned with when we are talking about sediment pollution, or sedimentation. This kind of sediment is usually from runoff during rainfall or snowmelt. This brings together timing, quantity, and type: these events are chronic, can deliver large quantities of fine sediment, and can occur all throughout the year.

This is a very complicated and extensively researched topic, but we hope this helps makes sense of the some of the nuances around sediment. It’s important to understand that some level of erosion and deposition, and the delivery of sediment to, and by, watercourses is natural and critical to the health of rivers and stream corridors while also acknowledging that sediment is one of the most pervasive pollutants of freshwater habitats. When things are shifted out of the natural range of variability, we can find ourselves in unhealthy situations.

Much of the sediment transport processes and problems can be observed in real time using a stream table, like the one on display in this video: https://www.youtube.com/watch?v=BuIYh17daSs

Here is another great YouTube video from MinuteEarth on why rivers meander, that does a great job explaining some of these ideas: https://www.youtube.com/watch?v=8a3r-cG8Wic

The nit and grit of sediment and streams

Sediment (soils, rocks, and other particles moving from the land into streams) is the focus of much discussion in the freshwater conservation world. How much is too much? When is sediment entering into streams a bad thing, or a good thing? There are a few important aspects of the sediment/water relationship that can help us better understand what is healthy or not, including:

Timing: when sediments are entering the stream, and how frequently they are entering the stream can shed light on whether it is a “natural” occurrence or not. Typically, sediment delivery is highest during the times of year when stream flows are highest. In SW Alberta, where this article was written, that’s usually spring and early summer. Our native fish evolved to deal with this by timing their spawning in the fall (like our chars – Bull Trout, Lake Trout) or in the spring (Westslope Cutthroat Trout) or summer (most minnow species) following high water, when the rate and volume of sediments ending up in the stream is low, or declining. Salmonids are particularly susceptible to sedimentation as their eggs are buried or settle into stream substrates (gravel etc.) and are prone to being “smothered” if there is too many fine sediments settling over these areas. Sedimentation outside the typical natural timing could be more impactful than during the usual high flow season for this reason.

The other aspect of timing is duration, “natural” sediment releases can be abrupt and can be quite extreme (e.g. landslides, bank failure, mass wasting, etc.), but they are also short term events that happen when streams are most able to manage this volume of material as flows are high and the sheer volume of water is able to transport this material onto floodplains or to downstream reaches where it becomes essential to the formation and maintenance of habitats. The other side of the sediment coin is that many watercourses, especially those which are heavily managed by dams, are actually sediment starved, cutting downward into the valley floor, experiencing significant erosion and disconnection with their floodplains which causes all sorts of ecological problems and loss of fish habitat. This is a common problem in urbanized watersheds, which can have a reduced supply of coarse sediments (gravels, cobbles) as streams are prevented from shifting sideways about their floodplain and eroding their banks (a natural process), and an increased supply of fine and very fine sediments (sand, silt) from roads, gravel lanes, construction sites, etc.

Quantity: the volume of sediment entering the stream is an important factor in how the stream deals with this material and whether it will have negative impacts. So many factors are interrelated – a little bit of sediment in the wrong place at the wrong time can be more impactful for some species than a large amount of sediment during the right time.

Type: sediment refers to a wide variety of materials from boulders all the way down to silts. When we see the brown rushing water in springtime, it’s usually because it’s full of fine sediments which are small particles able to be suspended in the current. Only during the highest flows can the largest streambed materials be moved; these are often referred to as channel-forming flows because the channel can shift and change as banks erode, while new banks are formed by deposited sediments.

Sediments are simultaneously responsible for spawning habitat creation (deposition of gravels and cobbles) and loss (deposition of fine sediments). Chronic delivery of fine sediments from human disturbances and landscapes is usually what we are concerned with when we are talking about sediment pollution, or sedimentation. This kind of sediment is usually from runoff during rainfall or snowmelt. This brings together timing, quantity, and type: these events are chronic, can deliver large quantities of fine sediment, and can occur all throughout the year.

This is a very complicated and extensively researched topic, but we hope this helps makes sense of the some of the nuances around sediment. It’s important to understand that some level of erosion and deposition, and the delivery of sediment to, and by, watercourses is natural and critical to the health of rivers and stream corridors while also acknowledging that sediment is one of the most pervasive pollutants of freshwater habitats. When things are shifted out of the natural range of variability, we can find ourselves in unhealthy situations.

Much of the sediment transport processes and problems can be observed in real time using a stream table, like the one on display in this video: https://www.youtube.com/watch?v=BuIYh17daSs

Here is another great YouTube video from MinuteEarth on why rivers meander, that does a great job explaining some of these ideas: https://www.youtube.com/watch?v=8a3r-cG8Wic

The nit and grit of sediment and streams

Sediment (soils, rocks, and other particles moving from the land into streams) is the focus of much discussion in the freshwater conservation world. How much is too much? When is sediment entering into streams a bad thing, or a good thing? There are a few important aspects of the sediment/water relationship that can help us better understand what is healthy or not, including:

Timing: when sediments are entering the stream, and how frequently they are entering the stream can shed light on whether it is a “natural” occurrence or not. Typically, sediment delivery is highest during the times of year when stream flows are highest. In SW Alberta, where this article was written, that’s usually spring and early summer. Our native fish evolved to deal with this by timing their spawning in the fall (like our chars – Bull Trout, Lake Trout) or in the spring (Westslope Cutthroat Trout) or summer (most minnow species) following high water, when the rate and volume of sediments ending up in the stream is low, or declining. Salmonids are particularly susceptible to sedimentation as their eggs are buried or settle into stream substrates (gravel etc.) and are prone to being “smothered” if there is too many fine sediments settling over these areas. Sedimentation outside the typical natural timing could be more impactful than during the usual high flow season for this reason.

The other aspect of timing is duration, “natural” sediment releases can be abrupt and can be quite extreme (e.g. landslides, bank failure, mass wasting, etc.), but they are also short term events that happen when streams are most able to manage this volume of material as flows are high and the sheer volume of water is able to transport this material onto floodplains or to downstream reaches where it becomes essential to the formation and maintenance of habitats. The other side of the sediment coin is that many watercourses, especially those which are heavily managed by dams, are actually sediment starved, cutting downward into the valley floor, experiencing significant erosion and disconnection with their floodplains which causes all sorts of ecological problems and loss of fish habitat. This is a common problem in urbanized watersheds, which can have a reduced supply of coarse sediments (gravels, cobbles) as streams are prevented from shifting sideways about their floodplain and eroding their banks (a natural process), and an increased supply of fine and very fine sediments (sand, silt) from roads, gravel lanes, construction sites, etc.

Quantity: the volume of sediment entering the stream is an important factor in how the stream deals with this material and whether it will have negative impacts. So many factors are interrelated – a little bit of sediment in the wrong place at the wrong time can be more impactful for some species than a large amount of sediment during the right time.

Type: sediment refers to a wide variety of materials from boulders all the way down to silts. When we see the brown rushing water in springtime, it’s usually because it’s full of fine sediments which are small particles able to be suspended in the current. Only during the highest flows can the largest streambed materials be moved; these are often referred to as channel-forming flows because the channel can shift and change as banks erode, while new banks are formed by deposited sediments.

Sediments are simultaneously responsible for spawning habitat creation (deposition of gravels and cobbles) and loss (deposition of fine sediments). Chronic delivery of fine sediments from human disturbances and landscapes is usually what we are concerned with when we are talking about sediment pollution, or sedimentation. This kind of sediment is usually from runoff during rainfall or snowmelt. This brings together timing, quantity, and type: these events are chronic, can deliver large quantities of fine sediment, and can occur all throughout the year.

This is a very complicated and extensively researched topic, but we hope this helps makes sense of the some of the nuances around sediment. It’s important to understand that some level of erosion and deposition, and the delivery of sediment to, and by, watercourses is natural and critical to the health of rivers and stream corridors while also acknowledging that sediment is one of the most pervasive pollutants of freshwater habitats. When things are shifted out of the natural range of variability, we can find ourselves in unhealthy situations.

Much of the sediment transport processes and problems can be observed in real time using a stream table, like the one on display in this video: https://www.youtube.com/watch?v=BuIYh17daSs

Here is another great YouTube video from MinuteEarth on why rivers meander, that does a great job explaining some of these ideas: https://www.youtube.com/watch?v=8a3r-cG8Wic