Surface Water, Reservoirs & Intakes

Learn surface-water source fundamentals, including rivers, lakes, reservoirs, intakes, seasonal stratification and turnover, algae, turbidity, debris, source variability, and operator troubleshooting.

Surface-water sources can change much more quickly than groundwater sources. Rivers, lakes, and reservoirs respond to weather, watershed runoff, seasonal temperature changes, biological activity, upstream discharges, and hydraulic conditions.

Operators must understand how source-water quality changes, how intakes interact with the source, and how reservoir conditions can affect the water entering the treatment plant.

Common Surface-Water Sources

Common surface-water sources include:

rivers;
streams;
lakes;
reservoirs.

Surface Water Is Highly Variable

Surface-water quality can change because of:

rainfall;
snowmelt;
drought;
temperature;
algal growth;
upstream spills;
construction;
agricultural runoff;
wastewater discharges.

Watershed Influence

A surface-water source reflects conditions throughout its watershed.

Changes in land use can affect:

runoff;
erosion;
nutrient loading;
organic matter;
microbial contamination.

River Sources

River sources can change rapidly because water is continuously moving downstream.

Operators may see fast changes in:

flow;
turbidity;
temperature;
conductivity;
organic matter.

High River Flow

High-flow conditions can result from:

heavy rainfall;
snowmelt;
upstream releases.

High flow may increase:

turbidity;
debris;
microbial loading;
organic matter;
intake velocity.

Low River Flow

Low-flow conditions can increase the influence of:

upstream discharges;
warm temperatures;
algal growth;
concentrated dissolved constituents.

Storm Runoff

Storm runoff can carry:

soil;
organic matter;
nutrients;
microorganisms;
road contaminants;
other pollutants.

Storm Travel Time

Changes in source water may appear after a delay between:

rainfall;
runoff entering the watershed;
arrival of changed water at the intake.

Lakes and Reservoirs

Lakes and reservoirs provide storage and can moderate some short-term changes.

However, they can develop complex seasonal water-quality patterns.

Reservoir Storage

Reservoir storage can provide:

water-supply reliability;
hydraulic buffering;
settling of some suspended material.

Reservoir Water Quality Can Vary with Depth

Temperature, dissolved oxygen, algae, iron, manganese, and other parameters can differ significantly between surface and deeper water.

Thermal Stratification

Thermal stratification occurs when water in a lake or reservoir separates into layers because of temperature and density differences.

Upper Layer

The warmer upper layer is commonly called the epilimnion.

Lower Layer

The colder deeper layer is commonly called the hypolimnion.

Transition Zone

The zone of rapid temperature change between the upper and lower layers is commonly called the thermocline.

Why Stratification Matters

Stratification limits vertical mixing.

Water quality in deeper layers can therefore become very different from surface water.

Dissolved Oxygen in Deep Water

During stratification, deeper water may lose dissolved oxygen because:

atmospheric oxygen cannot mix downward easily;
organic matter continues decomposing;
microorganisms consume oxygen.

Low-DO Bottom Water

Low dissolved oxygen can promote release of certain substances from sediments.

Operators may observe increased:

iron;
manganese;
odor compounds.

Iron and Manganese Release

Under reducing conditions, iron and manganese can become more soluble.

If deep reservoir water is withdrawn, these constituents can increase treatment demand.

Reservoir Turnover

Turnover occurs when temperature and density differences decrease enough for the water column to mix more completely.

Turnover Can Change Intake Water Quickly

During turnover, water from deeper layers can mix upward.

This can cause sudden changes in:

temperature;
iron;
manganese;
organic matter;
odor;
dissolved oxygen.

Operators Should Anticipate Seasonal Turnover

Historical reservoir data can help identify when turnover is likely and what treatment changes typically follow.

Algae

Algae can grow rapidly when conditions are favorable.

Important factors include:

sunlight;
temperature;
nutrients;
water residence time.

Algal Effects

Algae can contribute to:

taste and odor;
filter loading;
pH changes;
organic-matter increases;
higher disinfectant demand.

Algal Blooms

An algal bloom can develop rapidly and affect source-water quality over a short period.

Operators should trend:

temperature;
pH;
turbidity;
odor;
organic-matter indicators.

pH Changes from Algal Activity

Photosynthetic activity can increase pH during daylight by removing carbon dioxide from the water.

Daily pH patterns may therefore change during strong algal growth.

Taste and Odor

Some algae and microorganisms can produce compounds that create:

earthy odor;
musty odor;
other taste and odor concerns.

Surface-Water Turbidity

Turbidity can increase because of:

storm runoff;
bank erosion;
construction;
resuspension of sediment;
algal material.

Turbidity and Treatment

Higher raw-water turbidity can increase:

coagulant demand;
sludge production;
filter loading;
backwash frequency.

Rapid Turbidity Changes

Surface-water plants should be prepared for rapid raw-water turbidity changes during storms or upstream disturbances.

Natural Organic Matter

Natural organic matter can enter surface water from:

soil;
vegetation;
wetlands;
runoff.

Organic Matter and Treatment

Higher organic matter can increase:

coagulant demand;
chlorine demand;
disinfection byproduct precursor load.

Temperature

Surface-water temperature changes seasonally and can affect:

coagulation;
settling;
biological activity;
disinfection;
algal growth.

Cold Water

Cold water can slow:

chemical reactions;
floc formation;
settling.

Warm Water

Warm water can increase:

biological activity;
algal growth;
disinfectant decay.

Surface-Water Intakes

An intake withdraws raw water from a river, lake, or reservoir and delivers it to the treatment system.

Intake Components

Intake systems may include:

screens;
trash racks;
gates;
intake pipes;
pump stations;
multiple withdrawal elevations.

Intake Screens

Screens help prevent large debris from entering:

pumps;
piping;
treatment equipment.

Screen Fouling

Intake screens can be blocked by:

leaves;
branches;
aquatic vegetation;
ice;
trash;
biological growth.

Symptoms of Screen Blockage

Possible indicators include:

reduced intake flow;
higher differential head;
pump suction problems;
uneven pump operation.

Trash Racks

Trash racks remove larger debris before water reaches finer screens or pumps.

Debris After Storms

Storm events can suddenly increase:

branches;
leaves;
floating debris;
sediment.

Operators should inspect intake structures more frequently during these conditions.

Intake Pumping

Intake pumps must provide the required raw-water flow without operating outside acceptable suction conditions.

Low Source Level

Low river, lake, or reservoir level can affect:

pump submergence;
available suction head;
intake capacity.

High Source Level

Flood conditions can increase:

debris;
turbidity;
intake loading;
structural stress.

Intake Velocity

Velocity through an intake should remain within the intended design range.

Very high velocity can increase:

debris loading;
head loss;
hydraulic stress.

Multiple-Level Intakes

Reservoir intakes may have multiple withdrawal elevations.

This allows operators to select water from different depths.

Selective Withdrawal

Selective withdrawal means choosing an intake depth that provides more favorable water quality.

Operators may consider:

temperature;
dissolved oxygen;
iron;
manganese;
algae;
turbidity.

Example: Deep-Water Iron and Manganese

During stratification, deep water may become low in dissolved oxygen and develop higher soluble iron and manganese.

A shallower intake may provide better raw water if algal conditions are acceptable.

Example: Surface Algal Bloom

If surface water experiences heavy algal growth, a deeper intake may provide better water depending on reservoir conditions.

No Intake Level Is Always Best

The preferred withdrawal depth can change seasonally.

Operators should base selection on current source-water data rather than habit.

Intake Depth and Turbidity

Very low intake elevations may be affected by:

bottom sediment;
low dissolved oxygen;
iron and manganese.

Very shallow intakes may be affected by:

algae;
floating debris;
warm water.

Reservoir Level Changes

Changes in reservoir elevation can alter which water layer reaches a fixed intake.

An intake that normally draws mid-depth water may begin drawing closer to the surface or bottom as level changes.

Reservoir Residence Time

Long residence time can support:

particle settling;
algal growth;
temperature stratification;
water-quality aging.

Short Residence Time

Rapid reservoir turnover from high inflow can reduce settling and make reservoir quality behave more like river water.

Upstream Spills

Surface-water systems can be vulnerable to upstream:

fuel spills;
industrial releases;
transportation accidents;
wastewater releases.

Spill Travel Time

Operators should consider:

distance upstream;
river velocity;
flow conditions;
dilution.

Intake Shutdown or Source Change

Depending on the event and facility procedures, a system may need to:

change intake location;
use another source;
adjust treatment;
temporarily stop withdrawal.

Do Not Wait for the Contaminant to Reach the Plant

Early warning from upstream monitoring and communication can provide time for operational response.

Source-Water Monitoring Near Intakes

Useful measurements can include:

turbidity;
temperature;
pH;
conductivity;
dissolved oxygen;
organic-matter indicators;
algal observations.

Trend Intake Conditions

Historical trends help operators identify:

normal seasonal changes;
storm response;
turnover periods;
algal seasons;
unusual events.

Raw-Water Turbidity and Coagulant Demand

If turbidity rises, coagulant demand may increase, but the correct response should be based on actual treatment testing and process results.

Raw-Water Organic Matter and Chlorine Demand

An increase in organic matter can increase chlorine demand and may require treatment adjustments upstream of final disinfection.

Low Dissolved Oxygen

Low source-water DO, especially in deeper reservoir water, can be associated with:

iron;
manganese;
odor;
reducing conditions.

Reservoir Sampling by Depth

Depth-profile sampling can help operators understand:

temperature stratification;
DO profile;
iron and manganese changes;
algal distribution.

Surface-Water Microbial Risk

Surface water is generally more exposed to microbial contamination from:

wildlife;
livestock;
wastewater;
runoff;
human activity.

Particle Removal and Disinfection

Because microorganisms can be associated with particles, good clarification and filtration support effective disinfection.

Source Blending

Some plants blend:

multiple surface sources;
surface water and groundwater.

Blending can change:

turbidity;
temperature;
alkalinity;
organic matter;
chemical demand.

Intake Maintenance

Routine maintenance may include:

screen cleaning;
debris removal;
gate inspection;
pump inspection;
structural inspection.

Ice Conditions

Cold-weather sources may experience:

surface ice;
frazil ice;
intake blockage.

Frazil Ice

Frazil ice consists of small ice crystals that can accumulate on intake screens and reduce flow.

Aquatic Growth

Intake structures can also be affected by:

algae;
mussels;
other biological growth.

Sediment Accumulation

Sediment can accumulate near intake structures and reduce:

available opening;
effective depth;
hydraulic capacity.

Intake Inspection After Major Events

After floods or major storms, inspect for:

debris;
sediment;
damage;
changed source conditions.

Low-Flow Drought Conditions

During prolonged drought, operators may see:

lower reservoir level;
warmer water;
higher algal activity;
greater concentration of some dissolved constituents.

Water Quality Can Be More Important Than Maximum Intake Capacity

The best intake operating point is not always the one that provides the greatest hydraulic capacity.

Operators must also consider:

raw-water quality;
treatment capability;
source protection.

Common Surface-Water and Intake Mistakes

Assuming surface-water quality changes slowly.
Ignoring watershed weather and runoff.
Ignoring seasonal stratification and turnover.
Using one reservoir intake level throughout the year without checking water quality.
Ignoring intake-screen head loss.
Ignoring debris after storms.
Assuming high reservoir storage means constant source quality.
Ignoring low dissolved oxygen in deep water.
Ignoring travel time from upstream spills.
Changing treatment without confirming current source conditions.

A Practical River-Source Review

Review upstream weather and watershed conditions.
Review river flow.
Review raw-water turbidity.
Review temperature and conductivity.
Inspect intake screens and debris loading.
Review chemical demand and treatment response.
Compare with historical storm and seasonal patterns.

A Practical Reservoir Review

Review reservoir elevation.
Review temperature profile.
Review dissolved oxygen by depth where available.
Review algae and turbidity.
Review iron and manganese.
Compare available intake elevations.
Select the most appropriate withdrawal level according to facility procedures.
Trend treatment response.

A Practical Intake Problem Review

Verify source water level.
Inspect screens and trash racks.
Review differential head where available.
Check pump suction conditions.
Inspect for debris, ice, or biological fouling.
Review intake gate or valve positions.
Compare actual intake flow with expected flow.
Inspect for sediment accumulation or structural problems.

What to Remember for the Exam

Surface-water sources include rivers, streams, lakes, and reservoirs.
Surface-water quality can change rapidly after storms, spills, and seasonal events.
Storm runoff can increase turbidity, microorganisms, nutrients, organic matter, and debris.
Reservoirs can develop thermal stratification, with warmer surface water and colder deeper water.
Deep reservoir water can become low in dissolved oxygen during stratification.
Low-DO bottom water can contain higher soluble iron and manganese.
Reservoir turnover mixes water layers and can rapidly change intake quality.
Algal growth can affect taste, odor, pH, organic matter, and filter loading.
Raw-water turbidity affects coagulation, sludge production, and filter loading.
Natural organic matter can increase coagulant and disinfectant demand.
Intake screens and trash racks protect pumps and treatment equipment from debris.
Blocked screens can reduce intake flow and worsen pump suction conditions.
Multiple-level reservoir intakes allow selective withdrawal from different depths.
The best intake depth can change seasonally.
High flow can increase turbidity and debris, while low flow can increase the influence of upstream discharges and warm-water conditions.
Operators should consider travel time for upstream spills and storm effects.
Surface-water monitoring should include source conditions and intake operation, not only treatment-plant data.
Intake maintenance includes screen cleaning, debris removal, gate inspection, pump inspection, and structural checks.
Source blending can change treatment requirements even if total plant flow is unchanged.
Good surface-water operation connects watershed conditions, source-water trends, intake performance, and treatment response.